[Bug c++/67427] New: [concepts] Subsumption dependence on template parameter ordering

["Casey at Carter dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67427

            Bug ID: 67427
           Summary: [concepts] Subsumption dependence on template
                    parameter ordering
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

r227376 fails to compile this correct program with -std=gnu++1z:

template <class S, class I>
concept bool Sentinel =
  requires (I i) { i; };

Sentinel{S, I}
void distance(I first, S last) {}

template <class I, class S>
concept bool SizedIteratorRange =
  Sentinel<S, I> && true;

#if FOO
template <class S, class I>
#else
template <class I, class S>
#endif
  requires SizedIteratorRange<I, S>
void distance(I first, S last) {}

int main() {
  distance(42, 43);
}

with error:

~/gcc6-r227376/bin/g++ -std=gnu++1z ~/foo4.cpp -c
/home/casey/foo4.cpp: In function âint main()â:
/home/casey/foo4.cpp:21:18: error: call of overloaded âdistance(int, int)â is
ambiguous
   distance(42, 43);
                  ^
/home/casey/foo4.cpp:6:6: note: candidate: void distance(I, S) [with S = int; I
= int]
 void distance(I first, S last) {}
      ^
/home/casey/foo4.cpp:18:6: note: candidate: void distance(I, S) [with I = int;
S = int]
 void distance(I first, S last) {}
      ^

It does compile successfully when FOO is defined to 1, changing the declaration
order of the template parameters on the second overload of distance.