It turns out the semantics of the spec is somewhat awkward,
though theoretically nicer. It seems like we do need to change
the definition somewhat to make it less awkward, or at least
come up with a good workaround before adopting the spec.

We have introduce a small hack to mimic the old behavior for scalar
values.

Note that the value of p below is now 2 (default), but should
be the non-concrete 2 | int.

Proof:
    p: *((*1 | int) & 2)  | int   // substitution of both P conjuncts in p
    p: *(*_|_ | 2)  | int         // U1: distribute conjuncts
    p: *_|_ | 2  | int            // M2: remove mark
    p: 2  | int                   // value after removing default.

-- in.cue --
Q: *1 | int
q: *Q | int // 1 as expected

P: *1 | int
P: 2
p: *P | int // now 2, but should be (2 | int), according to the spec:

// Here the inner default may not be used as it is masked by the outer default.
r: (*3 | (*1 | 2)) & (1 | 2)

// Here the inner default is used, as there are no defaults marked in the
// outer disjunction.
s: (3 | (*1 | 2)) & (1 | 2)

s1: #Size & {min: 5}

#Size: {
	max: >min | *min
	res: uint | *0
	min: >res | *(1 + res)
}

staged: {
	c: ("a" | "b") & (*(*"a" | string) | string)
	d: (*(*"a" | string) | string) & ("a" | "b")
}

issue763a: {
	#A: {
		v: "a" | "b" | "c" // change to string to fix
	}

	h: [string]: #A

	h: [=~"^b"]: #A & {
		v: *h.a.v | string
	}

	h: a: {
		v: *"a" | string
	}

	h: baa: _
	h: boo: _
}
-- out/eval/stats --
Leaks:  0
Freed:  171
Reused: 158
Allocs: 13
Retain: 2

Unifications: 28
Conjuncts:    217
Disjuncts:    172
-- out/evalalpha --
(struct){
  Q: (int){ |(*(int){ 1 }, (int){ int }) }
  q: (int){ |(*(int){ 1 }, (int){ int }) }
  P: (int){ 2 }
  p: (int){ |(*(int){ 2 }, (int){ int }) }
  r: (int){ |((int){ 1 }, (int){ 2 }) }
  s: (int){ |(*(int){ 1 }, (int){ 2 }) }
  s1: (#struct){
    min: (int){ 5 }
    max: (number){ |(*(int){ 5 }, (number){ >5 }) }
    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
  }
  #Size: (#struct){
    max: (number){ |(*(int){ 1 }, (number){ >0 }, (number){ >1 }) }
    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
    min: (number){ |(*(int){ 1 }, (number){ >0 }) }
  }
  staged: (struct){
    c: (string){ |(*(string){ "a" }, *(string){ "b" }) }
    d: (string){ |(*(string){ "a" }, (string){ "b" }) }
  }
  issue763a: (struct){
    #A: (#struct){
      v: (string){ |((string){ "a" }, (string){ "b" }, (string){ "c" }) }
    }
    h: (struct){
      a: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
      baa: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
      boo: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
    }
  }
}
-- diff/-out/evalalpha<==>+out/eval --
diff old new
--- old
+++ new
@@ -6,9 +6,9 @@
   r: (int){ |((int){ 1 }, (int){ 2 }) }
   s: (int){ |(*(int){ 1 }, (int){ 2 }) }
   s1: (#struct){
-    max: (number){ |(*(int){ 5 }, (number){ >5 }) }
-    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
     min: (int){ 5 }
+    max: (number){ |(*(int){ 5 }, (number){ >5 }) }
+    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
   }
   #Size: (#struct){
     max: (number){ |(*(int){ 1 }, (number){ >0 }, (number){ >1 }) }
@@ -16,7 +16,7 @@
     min: (number){ |(*(int){ 1 }, (number){ >0 }) }
   }
   staged: (struct){
-    c: (string){ |(*(string){ "a" }, (string){ "b" }) }
+    c: (string){ |(*(string){ "a" }, *(string){ "b" }) }
     d: (string){ |(*(string){ "a" }, (string){ "b" }) }
   }
   issue763a: (struct){
-- diff/explanation --
The changes in default behavior as are shown here are according to spec, as is
described at the top of the file. These changes may pose too much of a problem
for the transition to the new evaluator, though.
UPDATE: we have made evalv3 a bit more in line with evalv2. Note how this
affects staged.c, though.
TODO(defaults): consider these cases with a potential default redesign.
-- diff/todo/p3 --
Reordering.
-- out/eval --
(struct){
  Q: (int){ |(*(int){ 1 }, (int){ int }) }
  q: (int){ |(*(int){ 1 }, (int){ int }) }
  P: (int){ 2 }
  p: (int){ |(*(int){ 2 }, (int){ int }) }
  r: (int){ |((int){ 1 }, (int){ 2 }) }
  s: (int){ |(*(int){ 1 }, (int){ 2 }) }
  s1: (#struct){
    max: (number){ |(*(int){ 5 }, (number){ >5 }) }
    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
    min: (int){ 5 }
  }
  #Size: (#struct){
    max: (number){ |(*(int){ 1 }, (number){ >0 }, (number){ >1 }) }
    res: (int){ |(*(int){ 0 }, (int){ &(>=0, int) }) }
    min: (number){ |(*(int){ 1 }, (number){ >0 }) }
  }
  staged: (struct){
    c: (string){ |(*(string){ "a" }, (string){ "b" }) }
    d: (string){ |(*(string){ "a" }, (string){ "b" }) }
  }
  issue763a: (struct){
    #A: (#struct){
      v: (string){ |((string){ "a" }, (string){ "b" }, (string){ "c" }) }
    }
    h: (struct){
      a: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
      baa: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
      boo: (#struct){
        v: (string){ |(*(string){ "a" }, (string){ "b" }, (string){ "c" }) }
      }
    }
  }
}
-- out/compile --
--- in.cue
{
  Q: (*1|int)
  q: (*〈0;Q〉|int)
  P: (*1|int)
  P: 2
  p: (*〈0;P〉|int)
  r: ((*3|(*1|2)) & (1|2))
  s: ((3|(*1|2)) & (1|2))
  s1: (〈0;#Size〉 & {
    min: 5
  })
  #Size: {
    max: (>〈0;min〉|*〈0;min〉)
    res: (&(int, >=0)|*0)
    min: (>〈0;res〉|*(1 + 〈0;res〉))
  }
  staged: {
    c: (("a"|"b") & (*(*"a"|string)|string))
    d: ((*(*"a"|string)|string) & ("a"|"b"))
  }
  issue763a: {
    #A: {
      v: ("a"|"b"|"c")
    }
    h: {
      [string]: 〈1;#A〉
    }
    h: {
      [=~"^b"]: (〈1;#A〉 & {
        v: (*〈2;h〉.a.v|string)
      })
    }
    h: {
      a: {
        v: (*"a"|string)
      }
    }
    h: {
      baa: _
    }
    h: {
      boo: _
    }
  }
}