@Part(06, Root="ada.mss")

@Comment{$Date: 2006/10/18 00:25:24 $}
@LabeledSection{Subprograms}

@Comment{$Source: e:\\cvsroot/ARM/Source/06.mss,v $}
@Comment{$Revision: 1.77 $}

@begin{Intro}
@Defn{subprogram}
@Defn{procedure}
@Defn{function}
A subprogram is a program unit or intrinsic operation whose execution
is invoked by a subprogram call.
There are two forms of subprogram: procedures and functions.
A procedure call is a @nt{statement}; a function call is an expression and
returns a value.
The definition of a subprogram can be given in two parts:
a subprogram declaration defining its interface,
and a @nt{subprogram_body} defining its execution.
@Redundant[Operators and enumeration literals are functions.]
@begin{Honest}
  A function call is an expression, but more specifically it is a @nt<name>.
@end{Honest}
@ChgToGlossary{Version=[2],Kind=[AddedNormal],Term=<Subprogram>,
Text=<@ChgAdded{Version=[2],Text=[A subprogram is a section of a program that
can be executed in various contexts. It is invoked by a subprogram call that
may qualify the effect of the subprogram through the passing of parameters.
There are two forms of subprograms: functions, which return values, and
procedures, which do not.]}>}
@ChgToGlossary{Version=[2],Kind=[AddedNormal],Term=<Function>,
Text=<@ChgAdded{Version=[2],Text=[A function is a form of subprogram that
returns a result and can be called as part of an expression.]}>}
@ChgToGlossary{Version=[2],Kind=[AddedNormal],Term=<Procedure>,
Text=<@ChgAdded{Version=[2],Text=[A procedure is a form of subprogram that
does not return a result and can only be called by a @nt{statement}.]}>}

@Defn{callable entity}
A @i(callable entity) is a subprogram or entry (see Section 9).
@Defn{call}
A callable entity is invoked by a @i{call};
that is, a subprogram call or entry call.
@Defn{callable construct}
A @i(callable construct) is a construct
that defines the action of a call upon a callable entity:
a @nt{subprogram_body},
@nt{entry_body}, or @nt{accept_statement}.
@begin{Ramification}
Note that @lquotes@;callable entity@rquotes@;
includes predefined operators, enumeration literals,
and abstract subprograms.
@lquotes@;Call@rquotes@; includes calls of these things.
They do not have callable constructs,
since they don't have completions.
@end{Ramification}
@end{Intro}

@LabeledClause{Subprogram Declarations}

@begin{Intro}
@Redundant[A @nt{subprogram_declaration} declares a procedure or
function.]
@end{Intro}

@begin{Syntax}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00218-03]}
@Syn{lhs=<subprogram_declaration>,rhs="@Chg{Version=[2],New=<
    [@Syn2{overriding_indicator}]
    >,Old=<>}@Syn2{subprogram_specification};"}

@ChgRef{Version=[2],Kind=[Deleted],ARef=[AI95-00348-01]}
@DeletedSyn{Version=[2],lhs=<@Chg{Version=[2],New=<>,Old=<abstract_subprogram_declaration>}>,
rhs="@Chg{Version=[2],New=<>,Old=<@Syn2{subprogram_specification} @key{is} @key{abstract};>}"}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
@Syn{lhs=<subprogram_specification>,rhs="@Chg{Version=[2],New=[
    @Syn2{procedure_specification}
  | @Syn2{function_specification}],Old=[
    @key{procedure} @Syn2{defining_program_unit_name} @Syn2{parameter_profile}
  | @key{function} @Syn2{defining_designator} @Syn2{parameter_and_result_profile}]}"}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00348-01]}
@AddedSyn{Version=[2],lhs=<@Chg{Version=[2],New=<procedure_specification>,Old=<>}>,
rhs="@Chg{Version=[2],New=<@key{procedure} @Syn2{defining_program_unit_name} @Syn2{parameter_profile}>,Old=<>}"}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00348-01]}
@AddedSyn{Version=[2],lhs=<@Chg{Version=[2],New=<function_specification>,Old=<>}>,
rhs="@Chg{Version=[2],New=<@key{function} @Syn2{defining_designator} @Syn2{parameter_and_result_profile}>,Old=<>}"}

@Syn{lhs=<designator>,rhs="[@Syn2{parent_unit_name} . ]@Syn2{identifier} | @Syn2{operator_symbol}"}

@Syn{lhs=<defining_designator>,
   rhs="@Syn2{defining_program_unit_name} | @Syn2{defining_operator_symbol}"}

@Syn{lhs=<defining_program_unit_name>,rhs="[@Syn2{parent_unit_name} . ]@Syn2{defining_identifier}"}

@begin{SyntaxText}
@Redundant[The optional @nt{parent_unit_name} is only allowed for
library units (see @RefSecNum{Compilation Units - Library Units}).]
@end{SyntaxText}

@Syn{lhs=<operator_symbol>,rhs="@Syn2{string_literal}"}

@begin{SyntaxText}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00395-01]}
The sequence of characters in an @nt{operator_symbol} shall @Chg{Version=[2],
New=[form a reserved
word, a delimiter, or compound delimiter that corresponds],Old=[correspond]} to
an operator belonging to one of the six @Chg{Version=[2],New=[categories],
Old=[classes]} of operators
defined in clause @RefSecNum{Operators and Expression Evaluation}@Chg{Version=[2],
New=[],Old=[(spaces are not allowed and the case of letters
is not significant)]}.

@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00395-01]}
  @ChgAdded{Version=[2],Text=[The @lquote@;sequence of characters@rquotes
  of the string literal of the operator is a technical term (see
  @RefSecNum{String Literals}), and does not include the surrounding quote
  characters. As defined in @RefSecNum{Lexical Elements, Separators, and Delimiters},
  lexical elements are @lquotes@;formed@rquotes from a sequence of characters.
  Spaces are not allowed, and upper and lower case is not significant. See
  @RefSecNum{Lexical Elements, Separators, and Delimiters} and
  @RefSecNum{Reserved Words} for rules related to the use of @ntf{other_format}
  characters in delimiters and reserved words.]}
@end{Reason}

@end{SyntaxText}

@Syn{lhs=<defining_operator_symbol>,rhs="@Syn2{operator_symbol}"}

@Syn{lhs=<parameter_profile>,rhs="[@Syn2{formal_part}]"}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01],ARef=[AI95-00318-02]}
@Syn{lhs=<parameter_and_result_profile>,rhs="@Chg{Version=[2],New=[
    ],Old=[]}[@Syn2{formal_part}] @key{return}@Chg{Version=[2],New=< [@Syn2{null_exclusion}]>,Old=<>} @Syn2{subtype_mark}@Chg{Version=[2],New=<
  | [@Syn2{formal_part}] @key{return} @Syn2{access_definition}>,Old=<>}"}

@Syn{lhs=<formal_part>,rhs="
   (@Syn2{parameter_specification} {; @Syn2{parameter_specification}})"}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01]}
@Syn{lhs=<parameter_specification>,rhs="
    @Syn2{defining_identifier_list} : @Syn2{mode} @Chg{Version=[2],New=<[@Syn2{null_exclusion}]>,Old=<>} @Syn2{subtype_mark} [:= @Syn2{default_expression}]
  | @Syn2{defining_identifier_list} : @Syn2{access_definition} [:= @Syn2{default_expression}]"}

@Syn{lhs=<mode>,rhs="[@key{in}] | @key{in} @key{out} | @key{out}"}
@end{Syntax}

@begin{Resolution}
@Defn2{Term=[formal parameter], Sec=(of a subprogram)}
A @i(formal parameter) is an object
@Redundant[directly visible within a @nt{subprogram_body}]
that represents the actual parameter passed to the subprogram in a
call;
it is declared by a @nt{parameter_specification}.
@PDefn2{Term=[expected type],
  Sec=(parameter @nt{default_expression})}
For a formal parameter,
the expected type for its @nt<default_expression>,
if any, is that of the formal parameter.
@IndexSee{Term=[parameter],See=[formal parameter]}
@end{Resolution}

@begin{Legality}
@Defn{parameter mode}
The @i(parameter mode) of a formal parameter conveys the direction of
information transfer with the actual parameter:
@key(in), @key(in out), or @key(out).
Mode @key(in) is the default,
and is the mode of a parameter defined by an @nt{access_definition}.
The formal parameters of a function, if any, shall have
the mode @key(in).
@begin{Ramification}
Access parameters are permitted.
This restriction to @b(in) parameters is primarily a methodological
restriction, though it also simplifies implementation for some compiler
technologies.
@end{Ramification}

A @nt{default_expression} is only allowed in a @nt{parameter_specification}
for a formal parameter of mode @key(in).

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
@PDefn2{Term=[requires a completion], Sec=(@nt{subprogram_declaration})}
@PDefn2{Term=[requires a completion], Sec=(@nt{generic_subprogram_declaration})}
A @nt{subprogram_declaration}
or a @nt{generic_subprogram_declaration}
requires a completion:
@Redundant[a body, a @nt<renaming_declaration>
(see @RefSecNum(Renaming Declarations)), or a @Chg{Version=[2],New=[@nt{pragma}],
Old=[@key(pragma)]} Import (see @RefSecNum{Interfacing Pragmas})].
@Redundant[A completion is not allowed
for an @nt<abstract_subprogram_declaration>@Chg{Version=[2],New=[ (see
@RefSecNum{Abstract Types and Subprograms}) or a
@nt{null_procedure_declaration} (see @RefSecNum{Null Procedures})],Old=[]}.]
@begin(Ramification)
  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
  Abstract subprograms @Chg{Version=[2],New=[and null procedures ],Old=[]}are
  not declared by
  @nt{subprogram_declaration}s, and so do not require completion.
  Protected subprograms are declared by @nt{subprogram_declaration}s,
  and so require completion.
  Note that an abstract subprogram is a subprogram,
  and a protected subprogram is a subprogram,
  but a generic subprogram is not a subprogram.
@end(Ramification)

A @nt{name} that denotes a formal parameter
is not allowed within the @nt{formal_part} in which it is declared,
nor within the @nt{formal_part} of a corresponding body or
@nt{accept_statement}.
@begin{Ramification}
By contrast,
@nt<generic_formal_parameter_declaration>s are visible to subsequent
declarations in the same @nt<generic_formal_part>.
@end{Ramification}
@end{Legality}

@begin{StaticSem}
@Defn{profile}
The @i(profile) of (a view of) a callable entity
is either a @nt{parameter_profile} or
@nt{parameter_and_result_profile}@Redundant[;
it embodies information about
the interface to that entity @em for example, the profile includes
information about parameters passed to the callable entity.
All callable entities have a profile @em enumeration literals,
other subprograms, and entries.
An access-to-subprogram type has a designated profile.]
Associated with a profile is a calling convention.
A @nt<subprogram_declaration> declares a procedure or a function, as
indicated by the initial reserved word, with name and profile as given by
its specification.

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01],ARef=[AI95-00318-02]}
@PDefn2{Term=[nominal subtype], Sec=(of a formal parameter)}
The nominal subtype of a formal parameter is
the subtype @Chg{Version=[2],New=[determined],Old=[denoted]} by
@Chg{Version=[2],New=[the optional @nt{null_exclusion} and ], Old=[]}the
@nt{subtype_mark}, or defined by the @nt{access_definition}, in the
@nt{parameter_specification}.@Chg{Version=[2],New=[ The nominal
subtype of a function result is the subtype
determined by the optional @nt{null_exclusion} and the @nt{subtype_mark}, or
defined by the @nt{access_definition}, in the
@nt{parameter_and_result_profile}.
@PDefn2{Term=[nominal subtype], Sec=(of a function result)}], Old=[]}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01],ARef=[AI95-00254-01],ARef=[AI95-00318-02]}
@Defn{access parameter}
An @i(access parameter) is a formal @key[in] parameter
specified by an @nt{access_definition}.
@Defn{access result type}
@Chg{Version=[2],New=[An @i(access result type) is a function result type specified by
an @nt{access_definition}.],Old=[]}
An access parameter @Chg{Version=[2],New=[or result type ],Old=[]}is of an
anonymous
@Chg{Version=[2],New=[access],Old=[general access-to-variable]} type (see
@RefSecNum{Access Types}). @Redundant[Access parameters
@Chg{Version=[2],New=[of an access-to-object type],Old=[]} allow dispatching
calls to be controlled by access values.@Chg{Version=[2],New=[ Access
parameters of an access-to-subprogram type permit calls to subprograms passed
as parameters irrespective of their accessibility level.],Old=[]}]

@begin{Discussion}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[Access result types have normal accessibility and
thus don't have any special properties worth noting here.]}
@end{Discussion}

@leading@keepnext@Defn2{Term=[subtypes], Sec=(of a profile)}
The @i(subtypes of a profile) are:
@begin{Itemize}
  For any non-access parameters, the nominal subtype of the parameter.

  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00254-01]}
  For any access parameters@Chg{Version=[2],New=[ of an access-to-object type],Old=[]},
  the designated subtype of the parameter type.

  @ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00254-01]}
  @ChgAdded{Version=[2],Text=[For any access parameters of an access-to-subprogram
  type, the subtypes of the profile of the parameter type.]}

  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01],ARef=[AI95-00318-02]}
  @Chg{Version=[2],New=[For any non-access result, the nominal subtype of the
  function result.],Old=[For any result, the result subtype.]}

  @ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
  @Chg{Version=[2],New=[For any access result type of an access-to-object type,
  the designated subtype of the result type.],Old=[]}

  @ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
  @Chg{Version=[2],New=[For any access result type of an access-to-subprogram
  type, the subtypes of the profile of the result type.],Old=[]}

@end{Itemize}

@Redundant[@Defn2{Term=[types], Sec=(of a profile)}
The @i{types of a profile} are the types of those subtypes.]

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
@Redundant[A subprogram declared by an
@nt<abstract_subprogram_declaration>
is abstract; a subprogram declared by a @nt<subprogram_declaration>
is not. See @RefSec{Abstract Types and Subprograms}.@Chg{Version=[2],New=[
Similarly, a procedure defined by a
@nt{null_procedure_declaration} is a null procedure; a procedure declared by
a @nt{subprogram_declaration} is not. See @RefSec{Null Procedures}.],Old=[]}]

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00218-03]}
@ChgAdded{Version=[2],Text=[@Redundant[An @nt{overriding_indicator} is used to
indicate whether overriding is intended. See @RefSec{Overriding Indicators}.]]}
@end{StaticSem}

@begin{RunTime}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
@PDefn2{Term=[elaboration], Sec=(subprogram_declaration)}@Chg{Version=[2],New=[],Old=[
@PDefn2{Term=[elaboration], Sec=(abstract_subprogram_declaration)}]}
The elaboration of a @nt{subprogram_declaration}@Chg{Version=[2],New=[],Old=[
or an @nt{abstract_subprogram_declaration}]} has no effect.
@end{RunTime}

@begin{Notes}
A @nt{parameter_specification} with several identifiers is equivalent
to a sequence of single @nt{parameter_specification}s, as explained
in @RefSecNum{Objects and Named Numbers}.

Abstract subprograms do not have bodies, and cannot be used
in a nondispatching call (see @RefSec{Abstract Types and Subprograms}).

The evaluation of @nt<default_expression>s is caused by certain
calls, as described in @RefSecNum{Parameter Associations}.
They are not evaluated during the elaboration of
the subprogram declaration.

Subprograms can be called recursively and can be called
concurrently from multiple tasks.
@end{Notes}

@begin{Examples}
@leading@keepnext@i{Examples of subprogram declarations:}
@begin{Example}
@key[procedure] Traverse_Tree;
@key[procedure] Increment(X : @key[in] @key[out] Integer);
@key[procedure] Right_Indent(Margin : @key[out] Line_Size);          --@RI{  see @RefSecNum{Integer Types}}
@key[procedure] Switch(From, To : @key[in] @key[out] Link);                --@RI{  see @RefSecNum{Incomplete Type Declarations}}

@key[function] Random @key[return] Probability;                      --@RI{  see @RefSecNum{Floating Point Types}}

@key[function] Min_Cell(X : Link) @key[return] Cell;                 --@RI{  see @RefSecNum{Incomplete Type Declarations}}
@key[function] Next_Frame(K : Positive) @key[return] Frame;          --@RI{  see @RefSecNum{Access Types}}
@key[function] Dot_Product(Left, Right : Vector) @key[return] Real;  --@RI{  see @RefSecNum{Array Types}}

@key[function] "*"(Left, Right : Matrix) @key[return] Matrix;        --@RI{  see @RefSecNum{Array Types}}
@end{Example}

@begin{wide}
@leading@keepnext@i{Examples of @key[in] parameters with default expressions:}
@end{wide}
@begin{Example}
@key[procedure] Print_Header(Pages  : @key[in] Natural;
            Header : @key[in] Line    :=  (1 .. Line'Last => ' ');  --@RI{  see @RefSecNum{Array Types}}
            Center : @key[in] Boolean := True);
@end{Example}
@end{Examples}

@begin{Extend83}
@Defn{extensions to Ada 83}
The syntax for @nt{abstract_subprogram_declaration} is added.
The syntax for @nt{parameter_specification} is revised to allow
for access parameters (see @RefSecNum{Access Types})

Program units that are library units may have
a @nt{parent_unit_name} to indicate the parent of a child
(see Section 10).
@end{Extend83}

@begin{DiffWord83}
We have incorporated the rules from
RM83-6.5, @lquotes@;Function Subprograms@rquotes@; here and in
@RefSec{Subprogram Bodies}

We have incorporated the definitions of RM83-6.6, @lquotes@;Parameter and Result
Type Profile - Overloading of Subprograms@rquotes@; here.

The syntax rule for @nt{defining_operator_symbol} is new.
It is used for the defining occurrence of an @nt{operator_symbol},
analogously to @nt{defining_identifier}.
Usage occurrences use the @nt{direct_name} or @nt{selector_name}
syntactic categories.
The syntax rules for @nt{defining_designator} and
@nt{defining_program_unit_name} are new.
@end{DiffWord83}

@begin{Extend95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00218-03]}
  @ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
  Subprograms now allow @nt{overriding_indicator}s for better error checking
  of overriding.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00231-01]}
  @ChgAdded{Version=[2],Text=[An optional @nt{null_exclusion} can be used in a
  formal parameter declaration. Similarly, an optional @nt{null_exclusion} can
  be used in a function result.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[The return type of a function can be an
  anonymous access type.]}
@end{Extend95}

@begin{DiffWord95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00254-01]}
  @ChgAdded{Version=[2],Text=[A description of the purpose of anonymous
  access-to-subprogram parameters and the definition of the profile of
  subprograms containing them was added.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
  @ChgAdded{Version=[2],Text=[Split the production for
  @nt{subprogram_specification} in order to make the declaration
  of null procedures (see @RefSecNum{Null Procedures}) easier.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
  @ChgAdded{Version=[2],Text=[Moved the @SyntaxTitle and
  @RuntimeTitle for @nt{abstract_subprogram_declaration} to
  @RefSecNum{Abstract Types and Subprograms}, so that the syntax
  and semantics are together. This also keeps abstract and null
  subprograms similar.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00395-01]}
  @ChgAdded{Version=[2],Text=[Revised to allow @ntf{other_format} characters
  in @nt{operator_symbol}s in the same way as the underlying constructs.]}
@end{DiffWord95}


@LabeledClause{Formal Parameter Modes}

@begin{Intro}
@Redundant[A @nt{parameter_specification} declares a formal parameter of
mode @key[in], @key[in out], or @key[out].]
@end{Intro}

@begin{StaticSem}
@Defn{pass by copy}
@Defn{by copy parameter passing}
@Defn{copy parameter passing}
@Defn{pass by reference}
@Defn{by reference parameter passing}
@Defn{reference parameter passing}
A parameter is passed either @i{by copy} or @i{by reference}.
@Redundant[When a parameter is passed by copy,
the formal parameter denotes a separate object from the
actual parameter, and any information transfer between the two occurs
only before and after executing the subprogram.
When a parameter is passed by reference,
the formal parameter denotes (a view of) the object denoted by the
actual parameter; reads and updates of the formal parameter directly
reference the actual parameter object.]

@Defn{by-copy type}
A type is a @i(by-copy type) if it is an elementary type,
or if it is a descendant of a private type whose full type is a
by-copy type.
A parameter of a by-copy type is passed by copy.

@leading@keepnext@Defn{by-reference type}
A type is a @i(by-reference type) if it
is a descendant of one of the following:
@begin(itemize)
  a tagged type;

  a task or protected type;

  a nonprivate type with the reserved word @b(limited) in its declaration;
  @begin{Ramification}
    A limited private type is by-reference only if it falls
    under one of the other categories.
  @end{Ramification}

  a composite type with a subcomponent of a by-reference type;

  a private type
  whose full type is a by-reference type.
@end(itemize)

A parameter of a by-reference type is passed by reference.
@Defn2{Term=[associated object], Sec=(of a value of a by-reference type)}
Each value of a by-reference type has an associated object.
For a parenthesized expression, @nt{qualified_expression},
or @nt{type_conversion}, this object is the one associated with the
operand.

@begin{Ramification}
By-reference parameter passing makes sense only if there is an
object to reference; hence, we define such an object for each
case.

Since tagged types are by-reference types, this implies that every value
of a tagged type has an associated object. This simplifies things,
because we can define the tag to be a property of the object, and not of
the value of the object, which makes it clearer that object tags never
change.

We considered simplifying things even more by making every value (and
therefore every expression) have an associated object. After all,
there is little semantic difference between a constant object and a
value.
However, this would cause problems for untagged types.
In particular, we would have to do a constraint check on every read of a
type conversion (or a renaming thereof) in certain cases.

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
We do not want this definition to depend on the view of the type;
privateness is essentially ignored for this definition.
Otherwise, things would be confusing (does the rule apply at the call
site, at the site of the declaration of the subprogram,
at the site of the
@Chg{Version=[2],New=[return statement],Old=[@nt{return_statement}]}?),
and requiring different calls to use different mechanisms would be an
implementation burden.

@RefSec{Shared Variable Control} says that a composite type with an
atomic or volatile subcomponent is a by-reference type,
among other things.

@Defn2{Term=[associated object], Sec=(of a value of a limited type)}
Every value of a limited by-reference type is the value of
one and only one limited object.
The @i{associated object} of a value of a limited by-reference type is
the object whose value it represents.
@Defn2{Term=[same value], Sec=(for a limited type)}
Two values of a limited by-reference type are the @i{same}
if and only if they represent the value of the same object.

We say @lquotes@;by-reference@rquotes@; above because
these statements are not always true for limited private types
whose underlying type is nonlimited (unfortunately).
@end{Ramification}

@PDefn{unspecified}
For parameters of other types,
it is unspecified whether the parameter
is passed by copy or by reference.
@begin{Discussion}
  There is no need to incorporate the discussion of AI83-00178,
  which requires pass-by-copy for certain kinds of actual parameters,
  while allowing pass-by-reference for others.
  This is because we explicitly indicate that a function
  creates an anonymous constant object for its result,
  unless the type is a return-by-reference type
  (see @RefSecNum{Return Statements}).
  We also provide a special dispensation for
  instances of Unchecked_Conversion to return by reference, even
  if the result type is not a return-by-reference type
  (see @RefSecNum{Unchecked Type Conversions}).
@end{Discussion}
@end{StaticSem}

@begin{Bounded}
@Defn{distinct access paths}
@Defn2{Term=[access paths],Sec=(distinct)}
@IndexSee{Term=[aliasing],See=(distinct access paths)}
@PDefn2{Term=(bounded error),Sec=(cause)}
If one @nt<name> denotes a part of a formal parameter,
and a second @nt<name> denotes a part of
a distinct formal parameter or an object that is not
part of a formal parameter, then the two @nt<name>s are
considered @i(distinct access paths).
If an object is of a type for which the parameter passing
mechanism is not specified, then it is a bounded error to
assign to the object via one access path,
and then read the value of the object via a distinct access path,
unless the first access path denotes a part of a formal parameter that
no longer exists at the point of the second access
@Redundant[(due to leaving the corresponding callable construct).]
@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)}
The possible consequences are that Program_Error is raised,
or the newly assigned value is read, or some old value of the object is read.
@begin{Discussion}
For example, if we call @lquotes@;P(X => Global_Variable, Y => Global_Variable)@rquotes@;,
then within P, the names @lquotes@;X@rquotes@;, @lquotes@;Y@rquotes@;, and @lquotes@;Global_Variable@rquotes@;
are all distinct access paths.
If Global_Variable's type is neither pass-by-copy nor pass-by-reference,
then it is a bounded error to assign to Global_Variable and
then read X or Y, since the language does not specify whether the
old or the new value would be read. On the other hand, if
Global_Variable's type is pass-by-copy, then the old value would
always be read, and there is no error. Similarly, if Global_Variable's
type is defined by the language to be pass-by-reference, then the
new value would always be read, and again there is no error.
@end{Discussion}
@begin{Reason}
We are saying @i(assign) here, not @i(update),
because updating any subcomponent is considered
to update the enclosing object.

The @lquotes@;still exists@rquotes@; part is so that a read after the subprogram returns
is OK.

If the parameter is of a by-copy type,
then there is no issue here
@em the formal is not a view of the actual.
If the parameter is of a by-reference type,
then the programmer may depend on updates through one access path
being visible through some other access path,
just as if the parameter were of an access type.
@end{Reason}
@begin{ImplNote}
The implementation can keep a copy in a register of a parameter
whose parameter-passing mechanism is not specified.
If a different access path is used to update the object (creating
a bounded error situation),
then the implementation can still use the value of the register,
even though the in-memory version of the object has been changed.
However, to keep the error properly bounded,
if the implementation chooses to read the in-memory version,
it has to be consistent -- it cannot then assume that something it has
proven about the register is true of the memory location.
For example, suppose the formal parameter is L,
the value of L(6) is now in a register, and L(6) is used
in an @nt{indexed_component} as in @lquotes@;A(L(6)) := 99;@rquotes@;,
where A has bounds 1..3.
If the implementation can prove that the value for L(6) in the register is in
the range 1..3, then it need not perform the constraint check if it
uses the register value.
However, if the memory value of L(6) has been changed to 4,
and the implementation uses that memory value,
then it had better not alter memory outside of A.

Note that the rule allows the implementation to pass a parameter by
reference and then keep just part of it in a register, or,
equivalently, to pass part of the parameter by reference and another
part by copy.
@end{ImplNote}
@begin{Reason}
@leading@;We do not want to go so far as to say that the mere presence
of aliasing is wrong.
We wish to be able to write the following sorts of things
in standard Ada:
@begin{Example}
@key[procedure] Move ( Source  : @key[in]  String;
                 Target  : @key[out] String;
                 Drop    : @key[in]  Truncation := Error;
                 Justify : @key[in]  Alignment  := Left;
                 Pad     : @key[in]  Character  := Space);
--@RI{ Copies elements from Source to Target (safely if they overlap)}
@end{Example}

This is from the standard string handling package.
It would be embarrassing if this couldn't be written in Ada!

The @lquotes@;then@rquotes@; before @lquotes@;read@rquotes@; in the rule implies that the implementation
can move a read to an earlier place in the code, but not to a later
place after a potentially aliased assignment.
Thus, if the subprogram reads one of its parameters into a local
variable, and then updates another potentially aliased one,
the local copy is safe @em it is known to have the old value.
For example, the above-mentioned Move subprogram can be implemented
by copying Source into a local variable before assigning into Target.

@leading@;For an @nt{assignment_statement} assigning one array parameter to another,
the implementation has to check which direction to copy
at run time, in general, in case the actual parameters are
overlapping slices.
For example:
@begin{Example}
@key[procedure] Copy(X : @key[in out] String; Y: String) @key[is]
@key[begin]
    X := Y;
@key[end] Copy;
@end{Example}

It would be wrong for the compiler to assume that X and Y
do not overlap (unless, of course, it can prove otherwise).
@end{Reason}
@end{Bounded}

@begin{Notes}
A formal parameter of mode @key(in) is a constant
view (see @RefSecNum{Objects and Named Numbers});
it cannot be updated within the @nt{subprogram_body}.
@end{Notes}

@begin{Extend83}
@Defn{extensions to Ada 83}
The value of an @key(out) parameter may be read.
An @key(out) parameter is treated like a declared
variable without an explicit initial expression.
@end{Extend83}

@begin{DiffWord83}
Discussion of copy-in for parts of out parameters is now
covered in @RefSec{Parameter Associations}.

The concept of a by-reference type is new to Ada 95.

We now cover in a general way in @RefSecNum{Operations of Discriminated Types}
the rule regarding erroneous execution when a discriminant
is changed and one of the parameters depends on the discriminant.
@end{DiffWord83}


@LabeledClause{Subprogram Bodies}

@begin{Intro}
@Redundant[A @nt{subprogram_body} specifies the execution of a
subprogram.]
@end{Intro}

@begin{Syntax}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00218-03]}
@Syn{lhs=<subprogram_body>,rhs="@Chg{Version=[2],New=<
    [@Syn2{overriding_indicator}]>,Old=<>}
    @Syn2{subprogram_specification} @key{is}
       @Syn2{declarative_part}
    @key{begin}
        @Syn2{handled_sequence_of_statements}
    @key{end} [@Syn2{designator}];"}

@begin{SyntaxText}
If a @nt{designator} appears at the end of a @nt{subprogram_body},
it shall repeat the @nt{defining_designator} of the @nt{subprogram_specification}.
@end{SyntaxText}
@end{Syntax}

@begin{Legality}
@Redundant[In contrast to other bodies,]
a @nt{subprogram_body} need not be the completion of a previous
declaration@Redundant[,
in which case the body declares the subprogram].
If the body is a completion, it shall be the completion of a
@nt{subprogram_declaration} or @nt{generic_subprogram_declaration}.
The profile of a @nt{subprogram_body} that completes a declaration
shall conform fully to that of the declaration.
@Defn2{Term=[full conformance],Sec=(required)}
@end{Legality}

@begin{StaticSem}
A @nt{subprogram_body} is considered a declaration.
It can either complete a previous declaration,
or itself be the initial declaration of the subprogram.
@end{StaticSem}

@begin{RunTime}
@PDefn2{Term=[elaboration], Sec=(non-generic subprogram_body)}
The elaboration of a non-generic
@nt{subprogram_body} has no other effect than to establish
that the subprogram can from then on be called without
failing the Elaboration_Check.
@begin{Ramification}
See @RefSecNum{Generic Bodies} for elaboration of a generic body.
Note that protected @ntf{subprogram_bodies} never get elaborated;
the elaboration of the containing @nt{protected_body}
allows them to be called without failing the Elaboration_Check.
@end{Ramification}

@PDefn2{Term=[execution], Sec=(subprogram_body)}
@Redundant[The execution of a @nt{subprogram_body} is invoked by a
subprogram call.]
For this execution the @nt{declarative_part} is elaborated, and the
@nt{handled_sequence_of_statements} is then executed.
@end{RunTime}

@begin{Examples}
@leading@keepnext@i{Example of procedure body:}
@begin{Example}
@key[procedure] Push(E : @key[in] Element_Type; S : @key[in] @key[out] Stack) @key[is]
@key[begin]
   @key[if] S.Index = S.Size @key[then]
      @key[raise] Stack_Overflow;
   @key[else]
      S.Index := S.Index + 1;
      S.Space(S.Index) := E;
   @key[end] @key[if];
@key[end] Push;
@end{Example}

@begin{Wide}
@leading@keepnext@i{Example of a function body:}
@end{Wide}
@begin{Example}
@key[function] Dot_Product(Left, Right : Vector) @key[return] Real @key[is]
   Sum : Real := 0.0;
@key[begin]
   Check(Left'First = Right'First @key[and] Left'Last = Right'Last);
   @key[for] J @key[in] Left'Range @key[loop]
      Sum := Sum + Left(J)*Right(J);
   @key[end] @key[loop];
   @key[return] Sum;
@key[end] Dot_Product;
@end{Example}
@end{Examples}

@begin{Extend83}
@Defn{extensions to Ada 83}
A @nt{renaming_declaration} may be used instead of a @nt{subprogram_body}.
@end{Extend83}

@begin{DiffWord83}
The syntax rule for @nt{subprogram_body} now uses the syntactic category
@nt{handled_sequence_of_statements}.

The @nt{declarative_part} of a @nt{subprogram_body} is now required;
that doesn't make any real difference,
because a @nt{declarative_part} can be empty.

We have incorporated some rules from RM83-6.5 here.

RM83 forgot to restrict the definition of elaboration of a
@nt{subprogram_body} to non-generics.
@end{DiffWord83}

@begin{DiffWord95}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00218-03]}
@Chg{Version=[2],New=[@nt{Overriding_indicator} is added to
@nt{subprogram_body}.],Old=[]}
@end{DiffWord95}


@LabeledSubClause{Conformance Rules}

@begin{Intro}
@Defn{conformance}
@SeeAlso{Primary=[conformance],Other=(type conformance)}
@SeeAlso{Primary=[conformance],Other=(mode conformance)}
@SeeAlso{Primary=[conformance],Other=(subtype conformance)}
@SeeAlso{Primary=[conformance],Other=(full conformance)}
@Redundant[When subprogram profiles are given in more than one place,
they are required to conform in one of four ways: type conformance, mode
conformance, subtype conformance, or full conformance.]
@end{Intro}

@begin{StaticSem}
@ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0011],ARef=[AI95-00117-01]}
@leading@Defn{convention}
@Defn{calling convention}
@Redundant[As explained in @RefSec{Interfacing Pragmas},
a @i{convention} can be specified for an entity.]
@Chg{New=[Unless this International Standard states otherwise, the default
convention of an entity is Ada.],Old=[]}
@Redundant[For a callable entity or access-to-subprogram type,
the convention is called the @i{calling convention}.]
The following conventions are defined by the language:
@begin{Itemize}
@Defn{Ada calling convention}
@Defn2{Term=[calling convention], Sec=(Ada)}
The default calling convention for any subprogram not listed below is
@i{Ada}.
@Redundant[A @nt{pragma} Convention, Import, or Export may be used to override
the default calling convention (see @RefSecNum{Interfacing Pragmas})].
@begin{Ramification}
See also the rule about renamings-as-body
in @RefSecNum{Subprogram Renaming Declarations}.
@end{Ramification}

@leading@Defn{Intrinsic calling convention}
@Defn2{Term=[calling convention], Sec=(Intrinsic)}
The @i{Intrinsic} calling convention represents
subprograms that are @lquotes@;built in@rquotes@; to the compiler.
The default calling convention is Intrinsic for the following:
@begin{InnerItemize}
  an enumeration literal;

  a "/=" operator declared implicitly due to
  the declaration of "=" (see @RefSecNum{Overloading of Operators});

  any other implicitly declared subprogram unless it is
  a dispatching operation of a tagged type;

  an inherited subprogram of a generic formal tagged type
  with unknown discriminants;
@begin{Reason}
@Comment{8652/0011 suggests that the reason for this rule be documented in
         the AARM.}
@ChgRef{Version=[1],Kind=[Added]}
@Chg{New=[Consider:],Old=[]}
@begin{Example}
@ChgRef{Version=[1],Kind=[Added]}
@Chg{New=[@key[package] P @key[is]
    @key[type] Root @key[is tagged null record];
    @key[procedure] Proc(X: Root);
@key[end] P;],Old=[]}

@ChgRef{Version=[1],Kind=[Added]}
@Chg{New=[@key[generic]
    @key[type] Formal(<>) @key[is new] Root @key[with private];
@key[package] G @key[is]
    ...
@key[end] G;],Old=[]}

@ChgRef{Version=[1],Kind=[Added]}
@ChgAdded{Version=[1],Text=[@key[package body] G @key[is]
    ...
    X: Formal := ...;
    ...
    Proc(X); -- @RI[This is a dispatching call in Instance, because]
             -- @RI[the actual type for Formal is class-wide.]
    ...
    -- @RI[Proc'Access would be illegal here, because it is of]
    -- @RI[convention Intrinsic, by the above rule.]
@key[end] G;]}

@ChgRef{Version=[1],Kind=[Added]}
@ChgAdded{Version=[1],Text=[@key[type] Actual @key[is new] Root @key[with] ...;
@key[procedure] Proc(X: Actual);
@key[package] Instance @key[is new] G(Formal => Actual'Class);
    -- @RI[It is legal to pass in a class-wide actual, because Formal]
    -- @RI[has unknown discriminants.]]}
@end{Example}

@ChgRef{Version=[1],Kind=[Added]}
@Chg{New=[Within Instance, all calls to Proc will be dispatching calls, so Proc doesn't
really exist in machine code, so we wish to avoid taking 'Access of it.
This rule applies to those cases where the actual type might be class-wide,
and makes these Intrinsic, thus forbidding 'Access.],Old=[]}
@end{Reason}


  an attribute that is a subprogram;

  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00252-01]}
  a subprogram declared immediately
  within a @nt{protected_body}@Chg{Version=[2],New=[;],Old=[.]}

  @ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00252-01],ARef=[AI95-00407-01]}
  @ChgAdded{Version=[2],Text=[any prefixed view of a subprogram (see
  @RefSecNum{Selected Components}).]}
  @begin{Reason}
    @ChgRef{Version=[2],Kind=[AddedNormal]}
    @ChgAdded{Version=[2],Text=[The profile of a prefixed view is
    different than the @lquotes@;real@rquotes profile of the subprogram
    (it doesn't have the first parameter), so we don't want to be able
    to take 'Access of it, as that would require generating a wrapper of
    some sort.]}
  @end{Reason}

@end{InnerItemize}

@NoPrefix@Redundant[The Access attribute is not allowed
for Intrinsic subprograms.]
@begin{Ramification}
  The Intrinsic calling convention really represents any number of
  calling conventions at the machine code level;
  the compiler might have a different instruction sequence for
  each intrinsic.
  That's why the Access attribute is disallowed.
  We do not wish to require the implementation to generate
  an out of line body for an intrinsic.

  Whenever we wish to disallow the Access attribute in order to ease
  implementation, we make the subprogram Intrinsic.
  Several language-defined subprograms have
  @lquotes@;@key[pragma] Convention(Intrinsic, ...);@rquotes@;.
  An implementation might actually implement this
  as @lquotes@;@key[pragma] Import(Intrinsic, ...);@rquotes@;,
  if there is really no body, and the implementation of the subprogram
  is built into the code generator.

  Subprograms declared in @ntf{protected_bodies} will generally have a
  special calling
  convention so as to pass along the identification of the
  current instance of the protected type.
  The convention is not @i(protected) since such local subprograms
  need not contain any @lquotes@;locking@rquotes@; logic since they are
  not callable via @lquotes@;external@rquotes@; calls;
  this rule prevents an access value designating such a subprogram
  from being passed outside the protected unit.

  The @lquotes@;implicitly declared subprogram@rquotes@; above refers to
  predefined operators (other than the "=" of a tagged type) and
  the inherited subprograms of
  untagged types.
@end{Ramification}

@Defn{protected calling convention}
@Defn2{Term=[calling convention], Sec=(protected)}
The default calling convention is @i{protected}
for a protected subprogram,
and for an access-to-subprogram type with
the reserved word @key(protected) in its definition.

@Defn{entry calling convention}
@Defn2{Term=[calling convention], Sec=(entry)}
The default calling convention is @i{entry} for an entry.

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00254-01],ARef=[AI95-00409-01]}
@ChgAdded{Version=[2],Text=[The calling convention for an
anonymous access-to-subprogram parameter
or anonymous access-to-subprogram result is @i<protected> if the reserved
word @key{protected} appears in its definition and otherwise is the convention
of the subprogram that contains the parameter.]}
@begin{Ramification}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[The calling convention for other
  anonymous access-to-subprogram types is Ada.]}
@end{Ramification}

@ChgRef{Version=[1],Kind=[Added],Ref=[8652/0011],ARef=[AI95-00117-01]}
@Chg{Version=[1],New=[@Redundant[If not specified above as Intrinsic, the calling convention for any
inherited or overriding dispatching operation of a tagged type is that of the
corresponding subprogram of the parent type.] The default calling convention
for a new dispatching operation of a tagged type is the convention of the type.],
Old=[]}
@begin{Reason}
@ChgRef{Version=[1],Kind=[Added]}
@ChgAdded{Version=[1],Text=[The first rule is officially stated in
@RefSecNum(Dispatching Operations of Tagged Types). The second is intended
to make interfacing to foreign OOP languages easier, by making the default
be that the type and operations all have the same convention.]}
@end{Reason}
@end{Itemize}

Of these four conventions, only Ada and Intrinsic are
allowed as a @SynI{convention_}@nt{identifier}
in a @nt{pragma} Convention, Import, or Export.
@begin{Discussion}
  The names of the @i{protected} and @i{entry} calling conventions
  cannot be used in the interfacing pragmas.
  Note that @key[protected] and @key[entry] are reserved words.
@end{Discussion}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00409-01]}
@Defn{type conformance}
@Defn2{Term=[profile],Sec=(type conformant)}
Two profiles
are @i{type conformant} if they have the same number of parameters,
and both have a result if either does, and corresponding
parameter and result types are the same,
or, for access parameters@Chg{Version=[2],New=[ or access results],Old=[]},
corresponding designated types are the same@Chg{Version=[2],New=[, or
corresponding designated profiles are type conformant],Old=[]}.
@IndexSee{Term=[type profile],See=(profile, type conformant)}
@begin{Discussion}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00409-01]}
For @Chg{Version=[2],New=[anonymous access-to-object],Old=[access]}
parameters, the designated types have to be the same for type
conformance, not the access types,
since in general each access parameter has its own anonymous access
type, created when the subprogram is called.
Of course, corresponding parameters have to be either both access
parameters or both not access parameters.

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00409-01]}
@ChgAdded{Version=[2],Text=[Similarly, for anonymous access-to-subprogram
parameters, the designated profiles of the types, not the types themselves,
have to be conformant.]}
@end{Discussion}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02],ARef=[AI95-00409-01]}
@Defn{mode conformance}
@Defn2{Term=[profile],Sec=(mode conformant)}
Two profiles are @i{mode conformant} if they are type-conformant, and
corresponding parameters have identical modes, and, for access
parameters@Chg{Version=[2],New=[ or access result types],Old=[]}, the
designated subtypes statically match@Chg{Version=[2],New=[, or the designated
profiles are subtype conformant.],Old=[]}
@PDefn2{Term=[statically matching],Sec=(required)}

@Defn{subtype conformance}
@Defn2{Term=[profile],Sec=(subtype conformant)}
Two profiles are @i{subtype conformant} if they are mode-conformant,
corresponding subtypes of the profile statically match,
and the associated calling conventions are the same.
The profile of a generic formal subprogram is not subtype-conformant
with any other profile.
@PDefn2{Term=[statically matching],Sec=(required)}

@begin{Ramification}
@PDefn{generic contract issue}
@end{Ramification}

@Defn2{Term=[full conformance], Sec=(for profiles)}
@Defn2{Term=[profile],Sec=(fully conformant)}
Two profiles are @i{fully conformant} if they
are subtype-conformant, and corresponding parameters
have the same names and have @nt<default_expression>s that
are fully conformant with one another.
@begin{Ramification}
Full conformance requires subtype conformance,
which requires the same calling conventions.
However, the calling convention of the declaration and body of a
subprogram or entry are always the same by definition.
@end{Ramification}

@leading@Defn2{Term=[full conformance], Sec=(for expressions)}
Two expressions are @i(fully conformant) if,
@Redundant[after replacing each use of an operator with the equivalent
@nt{function_call}:]
@begin{Itemize}
each constituent construct of one corresponds to an instance of the
same syntactic category in the other,
except that an expanded name may correspond
to a @nt{direct_name}
(or @nt{character_literal})
or to a different expanded name in the other; and

each @nt{direct_name}, @nt{character_literal}, and @nt{selector_name}
that is not part of the @nt{prefix} of an expanded name in one
denotes the same declaration as the corresponding
@nt{direct_name}, @nt{character_literal},
or @nt{selector_name} in the other; and
@begin{Ramification}
Note that it doesn't say @lquotes@;respectively@rquotes@;
because a @nt{direct_name} can correspond to a @nt{selector_name},
and vice-versa, by the previous bullet.
This rule allows the @nt{prefix} of an expanded name to be removed,
or replaced with a different @nt{prefix} that denotes a renaming of the
same entity.
However, it does not allow a @nt{direct_name} or @nt{selector_name} to
be replaced with one denoting a distinct renaming
(except for @nt{direct_name}s and @nt{selector_name}s in @nt{prefix}es
of expanded names).
Note that calls using operator notation
are equivalent to calls using prefix notation.

@leading@keepnext@;Given the following declarations:
@begin{Example}
@key[package] A @key[is]
    @key[function] F(X : Integer := 1) @key[return] Boolean;
@key[end] A;

@key[with] A;
@key[package] B @key[is]
    @key[package] A_View @key[renames] A;
    @key[function] F_View(X : Integer := 9999) @key[return] Boolean @key[renames] F;
@key[end] B;

@key[with] A, B; @key[use] A, B;
@key[procedure] Main @key[is] ...
@end{Example}

Within Main, the expressions @lquotes@;F@rquotes@;, @lquotes@;A.F@rquotes@;, @lquotes@;B.A_View.F@rquotes@;,
and @lquotes@;A_View.F@rquotes@; are all fully conformant with one another.
However, @lquotes@;F@rquotes@; and @lquotes@;F_View@rquotes@; are not fully conformant.
If they were, it would be bad news, since the two denoted views have
different @nt{default_expression}s.
@end{Ramification}

@ChgRef{Version=[1],Kind=[Added],Ref=[8652/0018],ARef=[AI95-00175-01]}
@Chg{New=[each @nt{attribute_designator} in one must be the same as the
corresponding @nt{attribute_designator} in the other; and],Old=[]}

each @nt{primary} that is a literal in one
has the same value as the corresponding literal in the other.
@begin{Ramification}
The literals may be written differently.
@end{Ramification}
@end{Itemize}
@begin{Ramification}
Note that the above definition makes full conformance a transitive
relation.
@end{Ramification}

@Defn2{Term=[full conformance], Sec=(for @nt{known_discriminant_part}s)}
Two @nt{known_discriminant_part}s are @i(fully conformant) if they have
the same number of discriminants, and
discriminants in the same positions
have the same names, statically matching subtypes,
and @nt{default_expression}s that are fully conformant with
one another.
@PDefn2{Term=[statically matching],Sec=(required)}

@Defn2{Term=[full conformance], Sec=(for @nt{discrete_subtype_definition}s)}
Two @nt<discrete_subtype_definition>s are @i(fully conformant) if they
are both @nt<subtype_indication>s or are both @nt<range>s,
the @nt<subtype_mark>s (if any) denote the same subtype, and
the corresponding @nt<simple_expression>s of the @nt<range>s (if any)
fully conform.
@begin{Ramification}
  In the @nt{subtype_indication} case, any ranges have to @i{be}
  corresponding; that is, two @nt{subtype_indication}s cannot conform
  unless both or neither has a @nt{range}.
@end{Ramification}
@begin{Discussion}
  This definition is used in @RefSec(Entries and Accept Statements)
  for the conformance required between the @nt<discrete_subtype_definition>s
  of an @nt<entry_declaration> for a family of entries and the
  corresponding @nt<entry_index_specification> of the @nt<entry_body>.
@end{Discussion}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00345-01],ARef=[AI95-00397-01]}
@ChgAdded{Version=[2],Text=[@Defn{prefixed view profile}
The @i<prefixed view profile> of a subprogram is the profile obtained by
omitting the first parameter of that subprogram. There is no prefixed view
profile for a parameterless subprogram. For the purposes of defining subtype
and mode conformance, the convention of a prefixed view profile is considered
to match that of either an entry or a protected operation.]}

@begin{Discussion}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[This definition is used to define how
  primitive subprograms of interfaces match operations in task and
  protected type definitions (see @RefSecNum{Task Units and Task Objects} and
  @RefSecNum{Protected Units and Protected Objects}).]}
@end{Discussion}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[The weird rule about conventions is pretty much
  required for synchronized interfaces to make any sense. There will be
  wrappers all over the place for interfaces anyway. Of course, this doesn't
  imply that entries have the same convention as protected operations.]}
@end{Reason}
@end{StaticSem}

@begin{ImplPerm}

An implementation may declare an operator declared in a language-defined
library unit to be intrinsic.

@end{ImplPerm}

@begin{Extend83}
@Defn{extensions to Ada 83}
The rules for full conformance are relaxed @em they are now based on
the structure of constructs, rather than the sequence of lexical
elements. This implies, for example, that "(X, Y: T)"
conforms fully with "(X: T; Y: T)",
and "(X: T)" conforms fully with "(X: @key[in] T)".
@end{Extend83}

@begin{DiffWord95}
  @ChgRef{Version=[2],Kind=[AddedNormal],Ref=[8652/0011],ARef=[AI95-00117-01]}
  @ChgAdded{Version=[2],Text=[@b<Corrigendum:> Clarified that the default
  convention is Ada. Also clarified that the convention of a primitive
  operation of a tagged type is the same as that of the type.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],Ref=[8652/0018],ARef=[AI95-00175-01]}
  @ChgAdded{Version=[2],Text=[@b<Corrigendum:> Added wording to ensure that two
  attributes conform only if they have the same @nt{attribute_designator}.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00252-01],ARef=[AI95-00254-01],ARef=[AI95-00407-01]}
  @ChgAdded{Version=[2],Text=[Defined the calling convention for anonymous
  access-to-subprogram types and for prefixed views of subprograms (see
  @RefSecNum{Selected Components}).]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[Defined the conformance of access result types
  (see @RefSecNum{Subprogram Declarations}).]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00345-01],ARef=[AI95-00397-01]}
  @ChgAdded{Version=[2],Text=[Defined the prefixed view profile of subprograms
  for later use.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00409-01]}
  @ChgAdded{Version=[2],Text=[Defined the conformance of anonymous
  access-to-subprogram parameters.]}
@end{DiffWord95}


@LabeledSubClause{Inline Expansion of Subprograms}

@begin{Intro}
@Redundant[Subprograms may be expanded in line at the call site.]
@end{Intro}

@begin{Syntax}
@begin{SyntaxText}
@leading@keepnext@PDefn2{Term=[program unit pragma], Sec=(Inline)}
@PDefn2{Term=[pragma, program unit], Sec=(Inline)}
The form of a @nt{pragma} Inline,
which is a program unit pragma (see @RefSecNum{Pragmas and Program Units}),
is as follows:
@end{SyntaxText}

@PragmaSyn`@key{pragma} @prag(Inline)(@Syn2{name} {, @Syn2{name}});'
@end{Syntax}

@begin{Legality}
The @nt{pragma} shall apply to one or more callable entities
or generic subprograms.
@end{Legality}

@begin{StaticSem}
If a @nt{pragma} Inline applies to a callable entity,
this indicates that inline expansion is desired for all calls
to that entity.
If a @nt{pragma} Inline applies to a generic subprogram,
this indicates that inline expansion is desired for all calls
to all instances of that generic subprogram.
@begin{Ramification}
@leading@;Note that inline expansion is desired no matter what
name is used in the call.
This allows one to request inlining for only one of several
overloaded subprograms as follows:
@begin{Example}
@key[package] IO @key[is]
   @key[procedure] Put(X : @key[in] Integer);
   @key[procedure] Put(X : @key[in] String);
   @key[procedure] Put(X : @key[in] Character);
@key[private]
   @key[procedure] Character_Put(X : @key[in] Character) @key[renames] Put;
   @key[pragma] Inline(Character_Put);
@key[end] IO;

@key[with] IO; @key[use] IO;
@key[procedure] Main @key[is]
   I : Integer;
   C : Character;
@key[begin]
   ...
   Put(C); --@RI{ Inline expansion is desired.}
   Put(I); --@RI{ Inline expansion is NOT desired.}
@key[end] Main;
@end{Example}
@end{Ramification}
@begin{Ramification}
The meaning of a subprogram can be changed by a @nt{pragma} Inline only
in the presence of failing checks
(see @RefSecNum{Exceptions and Optimization}).
@end{Ramification}
@end{StaticSem}

@begin{ImplPerm}
For each call,
an implementation is free to follow or to ignore the recommendation
expressed by the @nt{pragma}.
@begin{Ramification}
Note, in particular, that the recommendation
cannot always be followed for a recursive call,
and is often infeasible for entries.
Note also that the implementation can inline calls even
when no such desire was expressed by a pragma,
so long as the semantics of the program remains unchanged.
@end{Ramification}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00309-01]}
@Chg{Version=[2],New=[An implementation may allow a @nt{pragma} Inline that has
an argument which is a @nt{direct_name} denoting a @nt{subprogram_body} of the
same @nt{declarative_part}.],Old=[]}
@begin{Reason}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[This is allowed for Ada 83 compatibility. This is
only a permission as this usage is considered obsolescent.]}
@end{Reason}
@begin{Discussion}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[We only need to allow this in @nt{declarative_part}s,
because a body is only allowed in another body, and these all have
@nt{declarative_part}s.]}
@end{Discussion}
@end{ImplPerm}

@begin{Notes}
The @nt{name} in a @nt{pragma} Inline can denote
more than one entity in the case of overloading.
Such a @nt{pragma} applies to all of the denoted entities.
@end{Notes}

@begin{Incompatible83}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00309-01]}
  @ChgAdded{Version=[2],Text=[@Defn{incompatibilities with Ada 83}
  A pragma Inline cannot refer to a @nt{subprogram_body} outside of that
  body. The pragma can be given inside of the subprogram body. Ada 2005
  adds an @ImplPermName to allow this usage for compatibility (and
  Ada 95 implementations also can use this permission), but
  implementations do not have to allow such @nt{pragma}s.]}
@end{Incompatible83}

@begin{Extend83}
  @Defn{extensions to Ada 83}
  A @nt{pragma} Inline is allowed inside a @nt{subprogram_body} if there
  is no corresponding @nt{subprogram_declaration}.
  This is for uniformity with other program unit pragmas.
@end{Extend83}

@begin{Extend95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00309-01]}
  @ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
  @b[Amendment Correction:] Implementations are allowed to let @nt{Pragma}
  Inline apply to a @nt{subprogram_body}.]}
@end{Extend95}


@RMNewPage@Comment{For printed RM Ada 2005 only}
@LabeledClause{Subprogram Calls}

@begin{Intro}
@Defn{subprogram call}
A @i{subprogram call} is either a @nt{procedure_call_statement}
or a @nt{function_call};
@Redundant[it invokes the execution of the @nt{subprogram_body}.
The call specifies the association of the actual parameters, if any,
with formal parameters of the subprogram.]
@end{Intro}

@begin{Syntax}
@Syn{lhs=<procedure_call_statement>,rhs="
    @SynI{procedure_}@Syn2{name};
  | @SynI{procedure_}@Syn2{prefix} @Syn2{actual_parameter_part};"}


@Syn{lhs=<function_call>,rhs="
    @SynI{function_}@Syn2{name}
  | @SynI{function_}@Syn2{prefix} @Syn2{actual_parameter_part}"}

@Syn{lhs=<actual_parameter_part>,rhs="
    (@Syn2{parameter_association} {, @Syn2{parameter_association}})"}

@Syn{lhs=<parameter_association>,rhs="
   [@SynI{formal_parameter_}@Syn2{selector_name} =>] @Syn2{explicit_actual_parameter}"}

@Syn{lhs=<explicit_actual_parameter>,rhs="@Syn2{expression} | @SynI{variable_}@Syn2{name}"}

@begin{SyntaxText}
@Defn{named association}
@Defn{positional association}
A @nt{parameter_association} is @i{named} or @i{positional}
according to whether or not the
@SynI{formal_@!parameter_}@!@nt{selector_@!name} is specified.
Any positional associations shall precede any named associations.
Named associations are not allowed if the @nt{prefix} in
a subprogram call is an @nt{attribute_@!reference}.
@begin{Ramification}
This means that the formal parameter names used in
describing predefined attributes are
to aid presentation of their semantics, but are not intended
for use in actual calls.
@end{Ramification}
@end{SyntaxText}
@end{Syntax}

@begin{Resolution}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00310-01]}
The @nt{name} or @nt{prefix} given in a @nt{procedure_call_statement}
shall resolve to denote
a callable entity that is a procedure, or an entry renamed
as (viewed as) a procedure.
The @nt{name} or @nt{prefix} given in a @nt{function_call}
shall resolve to denote
a callable entity that is a
function.@Chg{Version=[2],New=[ The @nt{name} or @nt{prefix} shall not
resolve to denote an abstract subprogram unless it is also a
dispatching subprogram.],Old=[]}
@Redundant[When there is an @nt<actual_@!parameter_@!part>, the @nt<prefix>
can be an @nt<implicit_@!dereference> of an access-to-subprogram
value.]
@begin{Discussion}
@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00310-01]}
@ChgAdded{Version=[2],Text=[This rule is talking about dispatching operations
(which is a static concept) and not about dispatching calls (which is a
dynamic concept).]}
@end{Discussion}
@begin{Ramification}
The function can be an operator,
enumeration literal, attribute that is a function, etc.
@end{Ramification}

A subprogram call shall contain at most one association for each
formal parameter.
Each formal parameter without an association shall have a
@nt{default_expression} (in the profile of the view denoted
by the @nt<name> or @nt<prefix>).
This rule is an overloading rule
(see @RefSecNum{The Context of Overload Resolution}).
@end{Resolution}

@begin{RunTime}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00345-01]}
@PDefn2{Term=[execution], Sec=(subprogram call)}
For the execution of a subprogram call,
the @nt{name} or @nt{prefix} of the call is evaluated,
and each @nt{parameter_@!association} is evaluated
(see @RefSecNum{Parameter Associations}).
If a @nt{default_@!expression} is used,
an implicit @nt{parameter_@!association} is assumed for this rule.
These evaluations are done in an arbitrary order.
The @nt{subprogram_@!body} is then executed@Chg{Version=[2],New=[, or a call
on an entry or protected subprogram is performed (see
@RefSecNum{Dispatching Operations of Tagged Types})],Old=[]}.
Finally, if the subprogram completes normally, then after it is left,
any necessary assigning back of formal to actual parameters occurs
(see @RefSecNum{Parameter Associations}).

@begin{Discussion}
  The implicit association for a default is only for this run-time rule.
  At compile time, the visibility rules are applied to the default at
  the place where it occurs, not at the place of a call.
@end{Discussion}
@begin{Honest}
  If the subprogram is inherited, see @RefSec{Derived Types and Classes}.

  If the subprogram is protected, see @RefSec{Protected Subprograms and Protected Actions}.

  If the subprogram is really a renaming of an entry, see @RefSec{Entry Calls}.

  @ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00345-01]}
  @ChgAdded{Version=[2],Text=[If the subprogram is implemented by an entry or
  protected subprogram, it will be treated as a dispatching call to the
  corresponding entry (see @RefSec{Entry Calls}) or protected subprogram (see
  @RefSec{Protected Subprograms and Protected Actions}).]}

  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00348-01]}
  Normally, the @nt{subprogram_body} that is executed by the above rule
  is the one for the subprogram being called.
  For an enumeration literal,
  implicitly declared (but noninherited) subprogram,
  @Chg{Version=[2],New=[null procedure, ],Old=[]}or an attribute that is
  a subprogram, an implicit body is assumed.
  For a dispatching call,
  @RefSec{Dispatching Operations of Tagged Types}
  defines which @nt{subprogram_body} is executed.
@end{Honest}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00407-01]}
@ChgAdded{Version=[2],Text=[If the @nt{name} or @nt{prefix} of a subprogram
call denotes a prefixed view (see @RefSecNum{Selected Components}), the
subprogram call is equivalent to a call on the underlying subprogram, with the
first actual parameter being provided by the @nt{prefix} of the prefixed view
(or the Access attribute of this @nt{prefix} if the first formal parameter is
an access parameter), and the remaining actual parameters given by the
@nt{actual_parameter_part}, if any.]}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)}
The exception Program_Error is raised at the point of a
@nt{function_call} if the function
completes normally without executing a
@Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]}.
@begin{Discussion}
We are committing to raising the exception at the point
of call, for uniformity @em see AI83-00152.
This happens after the function is left, of course.

Note that there is no name for suppressing this check,
since the check imposes no time overhead and minimal
space overhead (since it can usually be statically eliminated
as dead code).
@end{Discussion}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00231-01]}
A @nt{function_call} denotes a constant, as defined in
@RefSecNum{Return Statements}; the nominal subtype of the constant is
given by the @Chg{Version=[2],New=[nominal],Old=[result]} subtype of the
function@Chg{Version=[2],New=[ result],Old=[]}.
@PDefn2{Term=[nominal subtype], Sec=(of the result of a @nt<function_call>)}
@PDefn2{Term=[constant], Sec=(result of a @nt<function_call>)}
@end{RunTime}

@begin{Examples}
@leading@keepnext@i{Examples of procedure calls:}
@begin{Example}
Traverse_Tree;                                               --@RI{  see @RefSecNum{Subprogram Declarations}}
Print_Header(128, Title, True);                              --@RI{  see @RefSecNum{Subprogram Declarations}}

Switch(From => X, To => Next);                               --@RI{  see @RefSecNum{Subprogram Declarations}}
Print_Header(128, Header => Title, Center => True);          --@RI{  see @RefSecNum{Subprogram Declarations}}
Print_Header(Header => Title, Center => True, Pages => 128); --@RI{  see @RefSecNum{Subprogram Declarations}}
@end{Example}

@begin{Wide}
@leading@keepnext@i{Examples of function calls:}
@end{Wide}
@begin{Example}
Dot_Product(U, V)   --@RI{  see @RefSecNum{Subprogram Declarations} and @RefSecNum{Subprogram Bodies}}
Clock               --@RI{  see @RefSecNum{Delay Statements, Duration, and Time}}
F.@key[all]               --@RI{  presuming F is of an access-to-subprogram type @em see @RefSecNum{Access Types}}
@end{Example}

@begin{Wide}
@leading@keepnext@i{Examples of procedures with default expressions:}
@end{Wide}
@begin{Example}
@key[procedure] Activate(Process : @key[in] Process_Name;
                   After   : @key[in] Process_Name := No_Process;
                   Wait    : @key[in] Duration := 0.0;
                   Prior   : @key[in] Boolean := False);

@key[procedure] Pair(Left, Right : @key[in] Person_Name := @key[new] Person);   --@RI{  see @RefSecNum{Incomplete Type Declarations}}
@end{Example}

@begin{Wide}
@leading@keepnext@i{Examples of their calls:}
@end{Wide}
@begin{Example}
Activate(X);
Activate(X, After => Y);
Activate(X, Wait => 60.0, Prior => True);
Activate(X, Y, 10.0, False);

Pair;
Pair(Left => @key[new] Person, Right => @key[new] Person);
@end{Example}
@end{Examples}

@begin{Notes}
If a @nt{default_expression} is used for two or more parameters in a
multiple @nt{parameter_@!specification}, the @nt{default_@!expression} is
evaluated once for each omitted parameter. Hence in the above
examples, the two calls of Pair are equivalent.
@end{Notes}

@begin{Examples}
@leading@keepnext@i{Examples of overloaded subprograms:}
@begin{Example}
@key[procedure] Put(X : @key[in] Integer);
@key[procedure] Put(X : @key[in] String);

@key[procedure] Set(Tint   : @key[in] Color);
@key[procedure] Set(Signal : @key[in] Light);
@end{Example}

@begin{Wide}
@leading@keepnext@i{Examples of their calls:}
@end{Wide}
@begin{Example}
Put(28);
Put("no possible ambiguity here");

Set(Tint   => Red);
Set(Signal => Red);
Set(Color'(Red));

--@RI{  Set(Red) would be ambiguous since Red may}
--@RI{  denote a value either of type Color or of type Light}
@end{Example}
@end{Examples}

@begin{DiffWord83}
We have gotten rid of parameters @lquotes@;of the form of a type conversion@rquotes@;
(see RM83-6.4.1(3)).
The new view semantics of @nt{type_conversion}s allows us to use
normal @nt{type_conversion}s instead.

We have moved wording about run-time semantics of parameter associations
to @RefSecNum{Parameter Associations}.

We have moved wording about raising Program_Error for a function
that falls off the end to here from RM83-6.5.
@end{DiffWord83}

@begin{Extend95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00310-01]}
  @ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
  Nondispatching abstract operations are no longer considered when
  resolving a subprogram call. That makes it possible to use @key{abstract}
  to @lquotes@;undefine@rquotes@; a predefined operation for an untagged type.
  That's especially helpful when defining custom arithmetic packages.]}
@end{Extend95}

@begin{DiffWord95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00231-01]}
  @ChgAdded{Version=[2],Text=[Changed the definition of the nominal subtype
  of a @nt{function_call} to use the nominal subtype wording of
  @RefSecNum{Subprogram Declarations}, to take into account
  @nt{null_exclusion}s and access result types.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00345-01]}
  @ChgAdded{Version=[2],Text=[Added wording to clarify that the meaning of
  a call on a subprogram @lquotes@;implemented by@rquotes an entry or
  protected operation is defined by
  @RefSecNum{Dispatching Operations of Tagged Types}.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00407-01]}
  @ChgAdded{Version=[2],Text=[Defined the meaning of a call on a prefixed
  view of a subprogram (see @RefSecNum{Selected Components}).]}
@end{DiffWord95}


@LabeledSubClause{Parameter Associations}

@begin{Intro}
@Redundant[@Defn{parameter passing}
A parameter association defines the association between an
actual parameter and a formal parameter.]
@end{Intro}

@begin{MetaRules}
The parameter passing rules for @b(out) parameters
are designed to ensure that the parts of a type that have
implicit initial values (see @RefSecNum{Object Declarations})
don't become @lquotes@;de-initialized@rquotes@; by
being passed as an @b(out) parameter.
@end{MetaRules}

@begin{Resolution}
The @SynI{formal_parameter_}@nt{selector_name} of a
@nt{parameter_@!association} shall resolve to denote a
@nt{parameter_@!specification} of the view being called.

@Defn2{Term=[actual parameter], Sec=(for a formal parameter)}
The @i(actual parameter) is either the @nt<explicit_actual_parameter>
given in a @nt<parameter_association> for a given
formal parameter, or the corresponding @nt<default_expression>
if no @nt<parameter_association> is given for the formal parameter.
@Defn2{Term=[expected type], Sec=(actual parameter)}
The expected type for an actual parameter is the
type of the corresponding formal parameter.
@begin{Honest}
  The corresponding @nt<default_expression> is the one of the
  corresponding formal parameter
  in the profile of the view denoted by the @nt<name> or
  @nt<prefix> of the call.
@end{Honest}

If the mode is @key(in),
the actual is interpreted as an @nt{expression};
otherwise, the actual is interpreted only as a @nt{name}, if possible.
@begin{Ramification}
  This formally resolves the ambiguity present in the syntax rule
  for @nt<explicit_actual_parameter>. Note that we don't actually require
  that the actual be a @nt<name> if the mode is not @key(in);
  we do that below.
@end{Ramification}
@end{Resolution}

@begin{Legality}
If the mode is @key(in out) or @key(out),
the actual shall be a @nt<name> that denotes a variable.
@begin{Discussion}
  We no longer need @lquotes@;or a
  @nt{type_conversion} whose argument is the @nt{name} of a variable,@rquotes@;
  because a @nt{type_conversion} is now a @nt{name}, and a
  @nt{type_conversion} of a variable is a variable.
@end{Discussion}
@begin{Reason}
  @leading@;The requirement that the actual be a (variable) @nt<name> is not
  an overload resolution rule, since
  we don't want the difference between @nt<expression> and
  @nt{name} to be used to resolve overloading.
  For example:
  @begin{Example}
procedure Print(X : @key[in] Integer; Y : @key[in] Boolean := True);
procedure Print(Z : @key[in out] Integer);
. . .
Print(3); --@RI{ Ambiguous!}
  @end{Example}

  The above call to Print is ambiguous even though the call is
  not compatible with the second Print which requires an actual
  that is a (variable) @nt<name> (@lquotes@;3@rquotes@; is an @nt<expression>, not
  a @nt<name>). This requirement is a legality rule, so overload
  resolution fails before it is considered, meaning that the call
  is ambiguous.
@end{Reason}

The type of the actual parameter associated with an access parameter
shall be convertible (see @RefSecNum{Type Conversions})
to its anonymous access type.
@PDefn2{Term=[convertible],Sec=(required)}
@end{Legality}

@begin{RunTime}
@leading@keepnext@PDefn2{Term=[evaluation], Sec=(parameter_association)}
For the evaluation of a @nt{parameter_association}:
@begin{Itemize}
The actual parameter is first evaluated.

For an access parameter,
the @nt{access_definition} is elaborated,
which creates the anonymous access type.

For a parameter @Redundant[(of any mode)] that is passed by reference
(see @RefSecNum{Formal Parameter Modes}),
a view conversion of the actual parameter to the nominal subtype
of the formal parameter is evaluated,
and the formal parameter denotes that conversion.
@PDefn2{Term=[implicit subtype conversion],Sec=(parameter passing)}
@begin{Discussion}
We are always allowing sliding, even for [@key(in)[ @key(out) by-reference
parameters.
@end{Discussion}

@Defn2{Term=[assignment operation], Sec=(during evaluation of a
@nt{parameter_association})}
For an @key(in) or @key(in out) parameter that is passed
by copy (see @RefSecNum{Formal Parameter Modes}),
the formal parameter object is created,
and the value of the actual parameter is converted to the
nominal subtype of the formal parameter
and assigned to the formal.
@PDefn2{Term=[implicit subtype conversion],Sec=(parameter passing)}
@begin{Ramification}
The conversion mentioned here is a value conversion.
@end{Ramification}

@leading@keepnext@;For an @key(out) parameter that is passed by copy,
the formal parameter object is created, and:
@begin(itemize)
  For an access type, the formal parameter is initialized
  from the value of the actual, without a constraint check;
@begin{Reason}
  This preserves the @MetaRulesName that an object of an access type
  is always initialized with a @lquotes@;reasonable@rquotes@; value.
@end{Reason}

  For a composite type with discriminants or
  that has implicit initial values for any subcomponents
  (see @RefSecNum{Object Declarations}),
  the behavior is as for an @key[in out] parameter
  passed by copy.
@begin{Reason}
  This ensures that no part of an object of such a type
  can become @lquotes@;de-initialized@rquotes@; by being part of an @b(out)
  parameter.
@end{Reason}
@begin{Ramification}
  This includes an array type whose component type is an access type,
  and a record type with a component that has a @nt{default_expression},
  among other things.
@end{Ramification}

  For any other type, the formal parameter is uninitialized.
  If composite, a view conversion of the actual
  parameter to the nominal subtype of the formal is evaluated
  @Redundant[(which might raise Constraint_Error)],
  and the actual subtype of the formal is that of the
  view conversion. If elementary, the actual subtype of the formal
  is given by its nominal subtype.
@begin{Ramification}
  This case covers scalar types, and composite types whose
  subcomponent's subtypes do not have any implicit initial
  values.
  The view conversion for composite types ensures that if the lengths
  don't match between an actual and a formal array parameter,
  the Constraint_Error is raised before the call, rather than after.
@end{Ramification}
@end(itemize)
@end{Itemize}

@PDefn2{Term=[constrained],Sec=(object)}
@PDefn2{Term=[unconstrained],Sec=(object)}
A formal parameter of mode @key[in out] or @key[out] with
discriminants is constrained if either its nominal subtype
or the actual parameter is constrained.

@Defn{parameter copy back}
@Defn{copy back of parameters}
@Defn{parameter assigning back}
@Defn{assigning back of parameters}
@Defn2{Term=[assignment operation], Sec=(during parameter copy back)}
After normal completion and leaving of a subprogram, for each @key(in out)
or @key(out) parameter that is passed by copy,
the value of the formal parameter is converted to
the subtype of the variable given as the actual parameter
and assigned to it.
@PDefn2{Term=[implicit subtype conversion],Sec=(parameter passing)}
These conversions and assignments occur in an arbitrary
order.
@begin{Ramification}
The conversions mentioned above during parameter passing might raise
Constraint_Error @em (see @RefSecNum{Type Conversions}).
@end{Ramification}
@begin{Ramification}
If any conversion or assignment as part of parameter passing
propagates an exception, the exception is raised at the place
of the subprogram call;
that is, it cannot be handled inside the @nt{subprogram_body}.
@end{Ramification}
@begin{TheProof}
Since these checks happen before or after executing the
@nt{subprogram_body}, the execution of the @nt{subprogram_body} does
not dynamically enclose them, so it can't handle the exceptions.
@end{TheProof}
@begin{Discussion}
The variable we're talking about is the one denoted by the
@i{variable_}@nt{name} given as the @nt{explicit_actual_parameter}.
If this @i{variable_}@nt{name} is a @nt{type_conversion},
then the rules in @RefSecNum{Type Conversions} for assigning to a view
conversion apply.
That is, if X is of subtype S1, and the actual is S2(X),
the above-mentioned conversion will convert to S2,
and the one mentioned in @RefSecNum{Type Conversions} will convert to
S1.
@end{Discussion}
@end{RunTime}

@begin{Extend83}
@Defn{extensions to Ada 83}
In Ada 95, a program can rely on the fact that passing an object as
an @key[out] parameter does not @lquotes@;de-initialize@rquotes@; any parts of the
object whose subtypes have implicit initial values.
(This generalizes the RM83 rule that required copy-in for parts that
were discriminants or of an access type.)
@end{Extend83}

@begin{DiffWord83}
We have eliminated the subclause on Default Parameters,
as it is subsumed by earlier clauses and subclauses.
@end{DiffWord83}


@LabeledClause{Return Statements}

@begin{Intro}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
A @Chg{Version=[2],New=[@nt{simple_@!return_@!statement} or
@nt{extended_@!return_@!statement} (collectively called a @i<return statement>)
@Defn{return statement}],Old=[@nt{return_statement}]} is used to
complete the execution of the
innermost enclosing @nt{subprogram_@!body},
@nt{entry_@!body}, or @nt{accept_@!statement}.
@end{Intro}

@begin{Syntax}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Syn{lhs=<@Chg{Version=[2],New=[simple_return_statement],Old=[return_statement]}>,rhs="@key{return} [@Syn2{expression}];"}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@AddedSyn{Version=[2],lhs=<@Chg{Version=[2],New=[extended_return_statement],Old=[]}>,
rhs="@Chg{Version=[2],New=<
    @key{return} @Syn2{defining_identifier} : [@Key{aliased}] @Syn2{return_subtype_indication} [:= @Syn2{expression}] [@Key{do}
        @Syn2{handled_sequence_of_statements}
    @key{end} @key{return}];>,Old=[]}"}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@AddedSyn{Version=[2],lhs=<@Chg{Version=[2],New=[return_subtype_indication],Old=[]}>,
rhs="@Chg{Version=[2],New=<@Syn2{subtype_indication} | @Syn2{access_definition}>,Old=[]}"}

@end{Syntax}

@begin{Resolution}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Chg{Version=[2],New=[@Defn2{Term=[result subtype], Sec=(of a function)}
The @i<result subtype> of a function is the subtype denoted by the
@nt<subtype_mark>, or defined by the @nt<access_definition>, after the reserved
word @key<return> in the profile of the function.@PDefn2{Term=[expected type],
Sec=(@nt{expression} of @nt<simple_@!return_@!statement>)}],Old=[@Defn{return expression}
The @nt{expression}, if any, of a @nt{return_statement} is called the
@i{return expression}.
@Defn2{Term=[result subtype], Sec=(of a function)}
The @i(result subtype) of a function is the subtype denoted by the
@nt{subtype_mark} after the reserved word @key(return) in the profile
of the function.@PDefn2{Term=[expected type], Sec=(return expression)}]}
The expected type for @Chg{Version=[2],New=[the @nt{expression}, if any, of a
@nt{simple_@!return_@!statement}],Old=[a return expression]} is the result type of
the corresponding function.@Chg{Version=[2],New=[
@PDefn2{Term=[expected type], Sec=(@nt{expression} of @nt{extended_return_statement})}
The expected type for the
@nt{expression} of an @nt{extended_return_statement} is that of the
@nt{return_@!subtype_@!indication}.],Old=[]}
@begin{Honest}
The same applies to generic functions.
@end{Honest}
@end{Resolution}

@begin{Legality}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Chg{Version=[2],New=[@Defn2{Term=[apply],
Sec=(to a callable construct by a return statement)}],
Old=[@Defn2{Term=[apply], Sec=(to a callable construct by a @nt{return_statement})}]}
A @Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]}
shall be within a callable construct,
and it @i{applies to} the innermost @Chg{Version=[2],New=[callable construct
or @nt{extended_return_statement} that contains it],Old=[one]}.
A @Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]} shall
not be within a body that is within the construct to which the
@Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]} applies.

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
A function body shall contain at least one
@Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]} that
applies to the function body,
unless the function contains @nt{code_statement}s.
A @Chg{Version=[2],New=[@nt{simple_@!return_@!statement}],Old=[@nt{return_@!statement}]} shall
include @Chg{Version=[2],New=[an @nt{expression}],Old=[a return expression]}
if and only if it applies to a function
body.@Chg{Version=[2],New=[ An @nt<extended_return_statement> shall apply to
a function body.],Old=[]}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
  The requirement that a function body has to have at least one
  @Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]}
  is a @lquotes@;helpful@rquotes@; restriction.
  There @Chg{Version=[2],New=[has],Old=[was]} been some interest in lifting
  this restriction, or allowing a raise statement to substitute for the
  @Chg{Version=[2],New=[return statement],Old=[@nt{return_@!statement}]}.
  However, there was enough interest in leaving it as is
  that we decided not to change it.
@end{Reason}
@begin{Ramification}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[A return statement can apply to an
  @nt{extended_return_statement}, so a @nt{simple_@!return_@!statement} without
  an @nt{expression} can be given in one. However, neither
  @nt{simple_@!return_@!statement} with an @nt{expression} nor an
  @nt{extended_return_statement} can be given inside an
  @nt{extended_return_statement}, as they must apply (directly) to a
  function body.]}
@end{Ramification}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Type=[Leading],Text=[For an
@nt{extended_@!return_@!statement} that applies to a function body:]}

@begin{Itemize}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[If the result subtype of the function is defined by a
@nt{subtype_mark}, the @nt{return_@!subtype_@!indication} shall be a
@nt{subtype_indication}. The type of the @nt{subtype_indication} shall be the
result type of the function. If the result subtype of the function is
constrained, then the subtype defined by the @nt{subtype_indication} shall also
be constrained and shall statically match this result subtype.
@PDefn2{Term=[statically matching],Sec=(required)}
If the result subtype of the function is unconstrained, then the subtype
defined by the @nt{subtype_indication} shall be a definite subtype, or there
shall be an @nt{expression}.]}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[If the result subtype of the function is defined
by an @nt{access_definition}, the @nt{return_@!subtype_@!indication} shall be an
@nt{access_definition}. The subtype defined by the @nt{access_definition} shall
statically match the result subtype of the function. The accessibility level of
this anonymous access subtype is that of the result subtype.]}

@end{Itemize}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Type=[Leading],Text=[For any return statement
that applies to a function body:]}

@begin{Itemize}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[If the result subtype of the function is limited,
then the @nt{expression} of the return statement (if any) shall be an
@nt{aggregate}, a function call (or equivalent use of an operator), or a
@nt{qualified_expression} or parenthesized expression whose operand is one of
these.]}
@begin{Discussion}
@ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[In other words, if limited, the @nt{expression}
  must produce a @lquotes@;new@rquotes@; object, rather than being the name
  of a preexisting object (which would imply copying).]}
@end{Discussion}

@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00416-01]}
@ChgAdded{Version=[2],Text=[If the result subtype of the function is class-wide,
the accessibility level of the type of the @nt{expression} of the return
statement shall not be statically deeper than that of the master that
elaborated the function body. If the result subtype has one or more
unconstrained access discriminants, the accessibility level of the anonymous
access type of each access discriminant, as determined by the
@nt{expression} of the @nt{simple_@!return_@!statement} or the
@nt{return_@!subtype_@!indication}, shall not be
statically deeper than that of the master that elaborated the function body.]}

@begin{Discussion}
@ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[We know that if the result type is class wide,
  then there must be an @nt{expression} of the return statement. Similarly, if
  the result subtype is unconstrained, then either the
  @nt{return_@!subtype_@!indication} (if any) is constrained, or there must be an
  @nt{expression}.]}
@end{Discussion}

@end{Itemize}

@end{Legality}

@begin{StaticSem}
@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[@PDefn2{Term=[return object], Sec=(extended_return_statement)}
Within an @nt{extended_return_statement}, the @i{return object} is declared
with the given @nt{defining_identifier}, with the nominal subtype defined by
the @nt{return_@!subtype_@!indication}.]}
@end{StaticSem}

@begin{RunTime}
@ChgRef{Version=[2],Kind=[Added],ARef=[AI95-00318-02],ARef=[AI95-00416-01]}
@ChgAdded{Version=[2],Text=[@PDefn2{Term=[execution], Sec=(extended_return_statement)}
For the execution of an @nt{extended_return_statement}, the
@nt{subtype_indication} or @nt{access_definition} is elaborated. This creates
the nominal subtype of the return object. If there is an @nt{expression}, it
is evaluated and converted to the nominal subtype (which might raise
Constraint_Error @em see @RefSecNum{Type Conversions}@PDefn2{Term=[implicit subtype conversion],Sec=(function return)});
the return object is created and the converted value is assigned to
the return object. Otherwise, the return object is created and initialized
by default as for a stand-alone object of its nominal subtype (see
@RefSecNum{Object Declarations}). If the nominal subtype is indefinite, the
return object is constrained by its initial value.@PDefn2{Term=[creation],Sec=[of a return object]}]}

@begin{Ramification}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[If the result type is controlled or has a
  controlled part, appropriate calls on Initialize or Adjust are performed
  prior to executing the @nt{handled_sequence_of_statements}, except when the
  initial expression is an @nt{aggregate} (which requires build-in-place
  with no call on Adjust).]}

  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[If the return statement is left without resulting
  in a return (for example, due to an exception propagated from the
  @nt{expression} or the @nt{handled_sequence_of_statements}, or a goto out of
  the @nt{handled_sequence_of_statements}), the return object is finalized
  prior to leaving the return statement.]}
@end{Ramification}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Chg{Version=[2],New=[@PDefn2{Term=[execution], Sec=(simple_@!return_@!statement)}],
Old=[@PDefn2{Term=[execution], Sec=(return_statement)}]}
For the execution of a @Chg{Version=[2],New=[@nt{simple_@!return_@!statement}],
Old=[@nt{return_statement}]}, the @nt{expression}
(if any) is first evaluated@Chg{Version=[2],New=[,],Old=[ and]} converted to
the result subtype@Chg{Version=[2],
New=[, and then is assigned to the anonymous @i{return object}.
@PDefn2{Term=[return object], Sec=(simple_@!return_@!statement)}],Old=[.]}
@PDefn2{Term=[implicit subtype conversion],Sec=(function return)}
@begin{Ramification}
The conversion might raise
Constraint_Error @em (see @RefSecNum{Type Conversions}).
@end{Ramification}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02],ARef=[AI95-00416-01]}
@Chg{Version=[2],New=[@Redundant[If the return object has any parts that are tasks, the
activation of those tasks does not occur until after the function returns (see
@RefSecNum{Task Execution - Task Activation}).]],
Old=[If the result type is class-wide, then
the tag of the result is the tag of the value
of the @nt<expression>.]}
@begin{TheProof}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[This is specified by the rules in
  @RefSecNum{Task Execution - Task Activation}.]}
@end{TheProof}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[Only the caller can know when task activations
  should take place, as it depends on the context of the call. If the function
  is being used to initialize the component of some larger object, then that
  entire object must be initialized before any task activations. Even after the
  outer object is fully initialized, task activations are still postponed until
  the @key{begin} at the end of the declarative part if the function is being
  used to initialize part of a declared object.]}
@end{Reason}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02],ARef=[AI95-00344-01]}
@ChgDeleted{Version=[2],Type=[Leading],Keepnext=[T],Text=[]}@ChgNote{A dummy
ChgDeleted to get conditional "Leading".}If
the result type @Chg{Version=[2],New=[of a function ],Old=[]}is a specific
tagged type@Chg{Version=[2],New=[, the tag of the return object is that
of the result type. If the result type is class-wide, the tag of the
return object is that of the value of the expression. A check is made that
the accessibility level of the type identified by the tag of the result is
not deeper than that of the master that elaborated the function body. If
this check fails, Program_Error is raised.@Defn2{Term=[Program_Error],
Sec=(raised by failure of run-time check)}
@IndexCheck{Accessibility_Check}],Old=[:]}

@begin{Ramification}
  @ChgNote{Moved from after paragraph 10}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[The first sentence is true even if the
  tag of the @nt{expression} is different, which
  could happen if the @nt{expression} were a view conversion or a
  dereference of an access value. Note that for a limited type, because
  of the restriction to @nt{aggregate}s and function calls (and no conversions),
  the tag will already match]}.
@end{Ramification}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[The first rule ensures
  that a function whose result type is
  a specific tagged type always returns an object whose tag is that of the
  result type. This is important for dispatching on controlling result, and
  allows the caller to allocate the appropriate amount of space to hold
  the value being returned (assuming there are no discriminants).]}

  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[The check prevents the returned object
  from outliving its type. Note that this check cannot fail for a specific
  tagged type, as the tag represents the function's type, which necessarily
  must be declared outside of the function.]}
@end{Reason}
@begin{NotIso}
@ChgAdded{Version=[2],Noprefix=[T],Noparanum=[T],Text=[@Shrink{@i<Paragraphs 9
through 20 were deleted.>}]}@Comment{This message should be deleted if the
paragraphs are ever renumbered.}
@end{NotIso}

@begin(itemize)
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[@IndexCheck{Tag_Check}
  If it is limited, then
  a check is made that the tag of the value of the return expression
  identifies the result type.
  @Defn2{Term=[Constraint_Error],Sec=(raised by failure of run-time check)}
  Constraint_Error is raised if this check fails.]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[If it is nonlimited, then
  the tag of the result is that of the result type.]}
@begin{Ramification}
  @ChgNote{These two notes were revised and moved up}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[This is true even if the
  tag of the return expression is different.]}
@end{Ramification}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[These rules ensure
  that a function whose result type is
  a specific tagged type always returns an object whose tag is that of the
  result type. This is important for dispatching on controlling result,
  and, if nonlimited,
  allows the caller to allocate the appropriate amount of space to hold
  the value being returned (assuming there are no discriminants).]}
@end{Reason}
@end(itemize)

@ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
@ChgDeleted{Version=[2],Type=[Leading],Keepnext=[T],
Text=[@Defn{return-by-reference type}
A type is a @i(return-by-reference) type if it
is a descendant of one of the following:]}
@begin(itemize)
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a tagged limited type;]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a task or protected type;]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a nonprivate type with the reserved word
  @b(limited) in its declaration;]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a composite type with a subcomponent
  of a return-by-reference type;]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a private type
  whose full type is a return-by-reference type.]}
@end(itemize)
@begin{Ramification}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg]}
  @ChgDeleted{Version=[2],Text=[
  The above rules are such that there are no "Ada 83" types other than
  those containing tasks that are return-by-reference. This helps
  to minimize upward incompatibilities relating to return-by-reference.]}
@end{Ramification}

@ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
@ChgDeleted{Version=[2],Type=[Leading],Text=[@IndexCheck{Accessibility_Check}
If the result type is a return-by-reference type,
then a check is made that the return expression is one of the
following:]}
@begin(itemize)

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a @nt{name} that denotes an object view
  whose accessibility level is not deeper than that of the master that
  elaborated the function body; or]}
  @begin{Discussion}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00316-01]}
  @ChgNote{This really wasn't in the previous version, but we don't want it
  in a version without deletions shown...}
  @ChgDeleted{Version=[2],Text=[This rule was unnecessarily confusing, and the
  parenthetical remark "(or a value with an associated object, see 6.2)"
  was added @em and then the entire concept was deleted.]}
  @end{Discussion}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
  @ChgDeleted{Version=[2],Text=[a parenthesized expression or
  @nt{qualified_expression} whose operand is one of these kinds of expressions.]}

@end(itemize)

@ChgRef{Version=[2],Kind=[DeletedNoDelMsg],ARef=[AI95-00318-02]}
@ChgDeleted{Version=[2],Text=[@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)}
The exception Program_Error is raised if this check fails.]}
@begin{Discussion}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg]}
  @ChgDeleted{Version=[2],Text=[Compare the definition of return-by-reference
  with that of by-reference.]}

  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg]}
  @ChgDeleted{Version=[2],Text=[The return-by-reference types are all limited types
  except those that are limited only because of a limited
  private type with a nonlimited untagged full type.]}
@end{Discussion}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[DeletedNoDelMsg]}
  @ChgDeleted{Version=[2],Text=[@PDefn{generic contract issue}
  This check can often be performed at compile time. It is
  defined to be a run-time check to avoid generic contract model
  problems. In a future version of the standard, we anticipate
  that function return of a local variable will be illegal for all
  limited types, eliminating the need for the run-time check
  except for dereferences of an access parameter.]}
@end{Reason}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02],ARef=[AI95-00402-01],ARef=[AI95-00416-01]}
@Chg{Version=[2],
New=[If the result subtype of a function has one or more unconstrained access
discriminants,
a check is made that the accessibility level of the anonymous access type
of each access discriminant, as determined by the @nt{expression} or the
@nt{return_@!subtype_@!indication} of the function,
is not deeper than that of the master that elaborated the
function body. If this check fails, Program_Error is raised.
@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)}
@IndexCheck{Accessibility_Check}],
Old=[For a function with a return-by-reference result type
the result is returned by reference;
that is, the function call denotes a constant view of the
object associated
with the value of the return expression.
@Defn2{Term=[assignment operation], Sec=(during execution of a
@nt{return_statement})}
For any other function, the result is returned by copy;
that is,
the converted value is assigned into an anonymous constant created at
the point of the @nt{return_statement}, and the function call denotes
that object.]}
@begin{Ramification}
  @ChgRef{Version=[2],Kind=[Deleted]}
  @ChgDeleted{Version=[2],Text=[The assignment operation does the necessary
  value adjustment, as described in
  @RefSec{User-Defined Assignment and Finalization}.
  @RefSecNum{Completion and Finalization}
  describes when the anonymous constant is finalized.]}
@end{Ramification}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[The check prevents the returned
  object (for a nonlimited type) from outliving the object designated by one
  of its discriminants. The check is made on the values of the discriminants,
  which may come from the @nt{return_@!subtype_@!indication} (if constrained),
  or the @nt{expression}, but it is never necessary to check both.]}
@end{Reason}

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Chg{Version=[2],New=[For the execution of an
@nt{extended_@!return_@!statement}, the
@nt{handled_@!sequence_@!of_@!statements} is executed. Within this
@nt{handled_@!sequence_@!of_@!statements}, the execution of a
@nt{simple_@!return_@!statement} that applies to the
@nt{extended_@!return_@!statement} causes a transfer of control that completes
the @nt{extended_@!return_@!statement}. Upon completion
of a return statement that applies to a callable construct],Old=[Finally]}, a
transfer of control is performed which completes the execution of the callable
construct@Chg{Version=[2],New=[], Old=[ to which the @nt{return_@!statement}
applies]}, and returns to the caller.

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
@ChgAdded{Version=[2],Text=[In the case of a function, the @nt{function_call}
denotes a constant view of the return object.]}
@end{RunTime}

@begin{ImplPerm}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00416-01]}
@ChgAdded{Version=[2],Text=[If the result subtype of a function is
unconstrained, and a call on the function is used to provide the initial value
of an object with a constrained nominal subtype, Constraint_Error may be raised
at the point of the call (after abandoning the execution of the function body)
if, while elaborating the @nt{return_@!subtype_@!indication} or
evaluating the @nt{expression} of a return statement that applies to the
function body, it
is determined that the value of the result will violate the constraint of the
subtype of this object.]}
@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[Without such a permission, it would be very
  difficult to implement @lquotes@;build-in-place@rquotes semantics. Such an
  exception is not handleable within the function, because in the
  return-by-copy case, the constraint check to verify that the result satisfies
  the constraints of the object being initialized happens after the function
  returns, and we want the semantics to change as little as possible when
  switching between return-by-copy and build-in-place. This implies further
  that upon detecting such a situation, the implementation may need to simulate
  a goto to a point outside any local exception handlers prior to raising the
  exception.]}
@end{Reason}
@begin{Ramification}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[This permission is allowed during the evaluation
  of the @nt{expression} of an @nt{extended_return_statement}, because the
  @nt{return_@!subtype_@!indication} may be unconstrained and the @nt{expression}
  then would provide the constraints.]}
@end{Ramification}
@end{ImplPerm}

@begin{Examples}
@leading@keepnext@i{Examples of return statements:}
@begin{Example}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@key[return];                         --@RI{ in a procedure body, }@nt{entry_body}@RI{,}@Chg{Version=[2],New=[
                                -- @nt{accept_statement}@RI{, or }@nt{extended_return_statement}],Old=[@RI{ or }@nt{accept_statement}]}

@key[return] Key_Value(Last_Index);   --@RI{ in a function body}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
@Chg{Version=[2],New=<@key[return] Node : Cell @key{do}           --@RI{ in a function body, see @RefSecNum{Incomplete Type Declarations} for Cell}
   Node.Value := Result;
   Node.Succ := Next_Node;
@key{end} @key{return};>,Old=<>}
@end{Example}
@end{Examples}

@begin{Incompatible83}
@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
@Defn{incompatibilities with Ada 83}
In Ada 95, if the result type of a function has a part that is a task,
then an attempt to return a local variable will raise Program_Error.
@Chg{Version=[2],New=[This is illegal in Ada 2005, see below. ],
Old=[]}In Ada 83, if a function returns a local variable containing a task,
execution is erroneous according to AI83-00867. However,
there are other situations where functions that return tasks
(or that return a variant record only one of whose variants
includes a task)
are correct in Ada 83 but will raise Program_Error according
to the new rules.

The rule change was made because there will be more types (protected types,
limited controlled types) in Ada 95 for which
it will be meaningless to return a local variable, and making
all of these erroneous is unacceptable.
The current rule was felt to be the simplest that kept
upward incompatibilities to situations involving returning tasks,
which are quite rare.
@end{Incompatible83}

@begin{DiffWord83}
This clause has been moved here from chapter 5,
since it has mainly to do with subprograms.

A function now creates an anonymous object.
This is necessary so that controlled types
will work.

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
We have clarified that a
@Chg{Version=[2],New=[return statement],Old=[@nt{return_statement}]}
applies to a callable construct, not to a callable entity.

@ChgRef{Version=[2],Kind=[Revised],ARef=[AI95-00318-02]}
There is no need to mention generics in the rules about where a
@Chg{Version=[2],New=[return statement],Old=[@nt{return_statement}]}
can appear and what it applies to;
the phrase @lquotes@;body of a subprogram or generic subprogram@rquotes@; is
syntactic, and refers exactly to @lquotes@;@nt{subprogram_body}@rquotes@;.
@end{DiffWord83}

@begin{Incompatible95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[@Defn{incompatibilities with Ada 95} The entire
  business about return-by-reference types has been dropped. Instead, the
  @nt{expression} of a return statement of a limited type can only be an
  @nt{aggregate} or @nt{function_call} (see @RefSecNum{Limited Types}). This
  means that returning a global object or @nt{type_conversion}, legal in Ada
  95, is now illegal. Such functions can be converted to use anonymous access
  return types by adding @key{access} in the function definition and return
  statement, adding .@key{all} in uses, and adding @key{aliased} in the object
  declarations. This has the advantage of making the reference return semantics
  much clearer to the casual reader.]}

  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[We changed these rules so that functions,
  combined with the new rules for limited types (@RefSecnum{Limited Types}),
  can be used as build-in-place constructors for limited types. This reduces
  the differences between limited and nonlimited types, which will
  make limited types useful in more circumstances.]}
@end{Incompatible95}

@begin{Extend95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
  The @nt{extended_return_statement} is new. This provides a name for
  the object being returned, which reduces the copying needed to return
  complex objects (including no copying at all for limited objects). It also
  allows component-by-component construction of the return object.]}
@end{Extend95}

@begin{DiffWord95}
  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[The wording was updated to support anonymous
  access return subtypes.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00318-02]}
  @ChgAdded{Version=[2],Text=[The term @lquotes@;return expression@rquotes@;
  was dropped because reviewers found it confusing when applied to the default
  @nt{expression} of an @nt{extended_return_statement}.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00344-01],ARef=[AI95-00416-01]}
  @ChgAdded{Version=[2],Text=[Added accessibility checks to class-wide
  return statements. These checks could not fail in Ada 95 (as all of the
  types had to be declared at the same level, so the tagged type would
  necessarily have been at the same level as the type of the
  object).]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00402-01],ARef=[AI95-00416-01]}
  @ChgAdded{Version=[2],Text=[Added accessibility checks to
  return statements for types with access discriminants. Since such
  types have to be limited in Ada 95, the @nt{expression} of a return statement
  would have been illegal in order for this check to fail.]}

  @ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00416-01]}
  @ChgAdded{Version=[2],Text=[Added an @ImplPermName allowing
  early raising of Constraint_Error if the result cannot fit in the ultimate
  object. This gives implementations more flexibility to do built-in-place
  returns, and is essential for limited types (which cannot be built in a
  temporary).]}
@end{DiffWord95}


@LabeledAddedSubClause{Version=[2],Name=[Pragma No_Return]}

@ChgNote{Putting @nt{pragma} in the syntax font is used for Assert and Pure,
at least.}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[A @nt{pragma} No_Return indicates that a
procedure cannot return normally@Redundant[; it may propagate an exception
or loop forever].]}
@begin{Discussion}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[@nt{Pragma} No_Deposit will have to wait for Ada 2017. :-)]}
@end{Discussion}

@begin{Syntax}
@begin{SyntaxText}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[The form of a @nt{pragma} No_Return, which is a
representation pragma (see @RefSecNum{Operational and Representation Items}),
is as follows:]}
@end{SyntaxText}

@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=`@AddedPragmaSyn`Version=[2],@key{pragma} @prag<No_Return>(@SynI{procedure_}@Syn2{local_name}{, @SynI{procedure_}@Syn2{local_name}});''}
@end{Syntax}

@begin{Legality}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[@Defn{non-returning}Each
@SynI{procedure_}@nt{local_name} shall denote one or
more procedures or generic procedures; the denoted entities are
@i<non-returning>. The @SynI{procedure_}@nt{local_name} shall not denote a
null procedure nor an instance of a generic unit.]}

@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[A null procedure cannot have the appropriate
  non-returning semantics, as it does not raise an exception or loop forever.]}
@end{Reason}

@begin{Ramification}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[The procedure can be abstract. The denoted
  declaration can be a @nt{renaming_declaration} if it obeys the usual rules
  for representation pragmas: the renaming has to occur immediately within the
  same declarative region as the renamed subprogram. If a non-returning
  procedure is renamed (anywhere) calls through the new name still have the
  non-returning semantics.]}
@end{Ramification}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[A return statement shall not apply to a
non-returning procedure or generic procedure.]}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[A procedure shall be non-returning if it overrides
a dispatching non-returning procedure.
@PDefn{generic contract issue}
In addition to the places where
@LegalityTitle normally apply (see @RefSecNum{Generic Instantiation}),
this rule applies also in the private part of an instance of a generic unit.]}

@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[This ensures that dispatching calls to
  non-returning procedures will, in fact, not return.]}
@end{Reason}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[If a renaming-as-body completes a non-returning
procedure declaration, then the renamed procedure shall be non-returning.]}

@begin{Reason}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[This ensures that no extra code is needed to
  implement the renames (that is, no wrapper is needed) as the body has
  the same property.]}
@end{Reason}

@end{Legality}

@begin{StaticSem}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[If a generic procedure is non-returning, then so
are its instances. If a procedure declared within a generic unit is
non-returning, then so are the corresponding copies of that procedure in
instances.]}
@end{StaticSem}

@begin{RunTime}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[If the body of a non-returning procedure completes
normally, Program_Error is raised at the point of the call.
@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)}]}
@end{RunTime}
@begin{Discussion}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[Note that there is no name for suppressing
  this check, since the check represents a bug, imposes no time overhead,
  and minimal space overhead (since it can usually be statically eliminated
  as dead code).]}
@end{Discussion}
@begin{ImplNote}
  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[If a non-returning procedure tries to return, we
  raise Program_Error. This is stated as happening at the call site, because we
  do not wish to allow the procedure to handle the exception (and then,
  perhaps, try to return again!). However, the expected run-time model is that
  the compiler will generate @key{raise} Program_Error at the end of the
  procedure body (but not handleable by the procedure itself), as opposed to
  doing it at the call site. (This is just like the typical run-time model for
  functions that fall off the end without returning a value). The reason is
  indirect calls: in P.@key{all}(...);, the compiler cannot know whether P
  designates a non-returning procedure or a normal one. Putting the @key{raise}
  Program_Error in the procedure's generated code solves this problem neatly.]}

  @ChgRef{Version=[2],Kind=[AddedNormal]}
  @ChgAdded{Version=[2],Text=[Similarly, if one passes a non-returning
  procedure to a generic formal parameter, the compiler cannot know this at
  call sites (in shared code implementations); the raise-in-body solution
  deals with this neatly.]}
@end{ImplNote}

@begin{Examples}
@begin{Example}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00433-01]}
@ChgAdded{Version=[2],Text=[@key(procedure) Fail(Msg : String);  --@RI[ raises Fatal_Error exception]
@key(pragma) No_Return(Fail);
   --@RI[ Inform compiler and reader that procedure never returns normally]]}
@end{Example}
@end{Examples}

@begin{Extend95}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00329-01],ARef=[AI95-00414-01]}
@ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
@nt{Pragma} No_Return is new.]}
@end{Extend95}


@LabeledClause{Overloading of Operators}

@begin{Intro}
@Defn{operator}
@Defn{user-defined operator}
@Defn2{Term=[operator], Sec=(user-defined)}
An @i{operator} is a function whose @nt{designator} is an
@nt<operator_symbol>.
@Redundant[Operators, like other functions, may be overloaded.]
@end{Intro}

@begin{Resolution}
Each use of a unary or binary operator is equivalent to a
@nt{function_call} with @SynI{function_}@nt{prefix} being the
corresponding @nt{operator_symbol}, and with (respectively) one or
two positional actual parameters being the operand(s) of the operator
(in order).
@begin{Honest}
We also use the term operator
(in Section 4 and in @RefSecNum{Subprogram Declarations})
to refer to one of the syntactic categories
defined in @RefSec{Operators and Expression Evaluation}
whose names end with @lquotes@;_operator:@rquotes@;
@nt<logical_@!operator>,
@nt<relational_@!operator>,
@nt<binary_@!adding_@!operator>,
@nt<unary_@!adding_@!operator>,
@nt<multiplying_@!operator>, and
@nt<highest_@!precedence_@!operator>.
@end{Honest}
@end{Resolution}

@begin{Legality}
The @nt{subprogram_specification} of a unary or binary operator shall have
one or two parameters, respectively.
A generic function instantiation whose @nt{designator} is an
@nt{operator_symbol} is only allowed if the specification of the
generic function has the corresponding number of parameters.

@nt{Default_expression}s are not allowed for the parameters of an operator
(whether the operator is declared with an explicit
@nt{subprogram_specification} or by a @nt{generic_instantiation}).

An explicit declaration of "/=" shall not have a result
type of the predefined type Boolean.
@end{Legality}

@begin{StaticSem}
A declaration of "=" whose result type is
Boolean implicitly declares a declaration of "/=" that
gives the complementary result.
@end{StaticSem}

@begin{Notes}
The operators "+" and "@en@;" are both unary and binary operators,
and hence may be overloaded with both one- and two-parameter functions.
@end{Notes}

@begin{Examples}
@leading@keepnext@i{Examples of user-defined operators:}
@begin{Example}
@key[function] "+" (Left, Right : Matrix) @key[return] Matrix;
@key[function] "+" (Left, Right : Vector) @key[return] Vector;
@Comment{Blank line}
--@RI{  assuming that A, B, and C are of the type Vector}
--@RI{  the following two statements are equivalent:}
@Comment{Blank line}
A := B + C;
A := "+"(B, C);
@end{Example}
@end{Examples}

@begin{Extend83}
@Defn{extensions to Ada 83}
Explicit declarations of "=" are now permitted for any
combination of parameter and result types.

Explicit declarations of "/=" are now permitted, so long
as the result type is not Boolean.
@end{Extend83}


@RMNewPage@Comment{For printed RM Ada 2005}
@LabeledAddedClause{Version=[2],Name=[Null Procedures]}

@begin{Intro}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@ChgAdded{Version=[2],Text=[A @nt<null_procedure_declaration> provides a shorthand
to declare a procedure with an empty body.]}
@end{Intro}

@begin{Syntax}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@AddedSyn{Version=[2],lhs=<@Chg{Version=[2],New=<null_procedure_declaration>,Old=<>}>,
rhs="@Chg{Version=[2],New=<
   [@Syn2{overriding_indicator}]
   @Syn2{procedure_specification} @key{is} @key{null};>,Old=<>}"}
@end{Syntax}

@begin{StaticSem}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@ChgAdded{Version=[2],Text=[A @nt<null_procedure_declaration> declares a @i<null
procedure>.@Defn{null procedure}@Defn2{Term=[procedure],Sec=[null]}
A completion is not allowed for a @nt<null_procedure_declaration>.]}
@begin{Reason}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[There are no null functions because the return
value has to be constructed somehow; a function that always raises
Program_Error doesn't seem very useful or worth the complication.]}
@end{Reason}
@end{StaticSem}

@begin{RunTime}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@ChgAdded{Version=[2],Text=[The execution of a null procedure is invoked by a subprogram call.
For the execution of a subprogram call on a null procedure, the execution of
the @nt<subprogram_body> has no effect.]}
@begin{Ramification}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[Thus, a null procedure is equivalent to the body]}
@begin{Example}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[@key{begin}
   @key{null};
@key{end};]}
@end{Example}
@ChgRef{Version=[2],Kind=[AddedNormal]}
@ChgAdded{Version=[2],Text=[with the exception that a null procedure can be used in
place of a procedure specification.]}
@end{Ramification}

@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@ChgAdded{Version=[2],Text=[@PDefn2{Term=[elaboration], Sec=(null_procedure_declaration)}
The elaboration of a @nt{null_procedure_declaration} has no effect.]}
@end{RunTime}

@begin{Examples}
@begin{Example}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00433-01]}
@ChgAdded{Version=[2],Text=[@key(procedure) Simplify(Expr : @key(in out) Expression) @key(is null); --@RI[ see @RefSecNum{Tagged Types and Type Extensions}]
--@RI[ By default, Simplify does nothing, but it may be overridden in extensions of Expression]]}
@end{Example}
@end{Examples}

@begin{Extend95}
@ChgRef{Version=[2],Kind=[AddedNormal],ARef=[AI95-00348-01]}
@ChgAdded{Version=[2],Text=[@Defn{extensions to Ada 95}
Null procedures are new.]}
@end{Extend95}