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The bodies of tt(requires) declarations contain define constraints to apply to
template parameters. There are four types of requirements:
itemization(
it() em(Simple requirements): constraints on em(facilities) (like
requiring the availability of tt(operator+));
it() em(Type requirements): requiring the availability of (sub)types (like
the sub-type tt(value_type)) which must be available when using
standard tt(push_back) functions;
it() em(Compound requirements): requiring types resulting from applying
operators or returned by called (member) functions;
it() em(Nested requirements): defining concepts in terms of already
existing concepts.
)
Constraints must be compile-time verifiable.
When multiple constraints are specified, they must em(all) be compile-time
verifiable, and an actual type is only accepted by the compiler if all
requirements could be satisfied.
COMMENT(
Multiple constraints
can be defined by specifying multiple requirements in
tt(requires)-specifications and/or by
combining constraints using the boolean operators tt(and, or) and tt(not).
END)
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