[Bug c++/67934] New: [concepts] ICE when providing default function implementations using concepts

["michel.steuwer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67934

            Bug ID: 67934
           Summary: [concepts] ICE when providing default function
                    implementations using concepts
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michel.steuwer at gmail dot com
  Target Milestone: ---

Created attachment 36483
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36483&action=edit
Contains test case with preprocessed file.

The following code fails with an ICE:

===
#include <iostream>

template <typename T>
concept bool
JustEqualityComparable() {
  return requires(T a, T b) {
    { a == b } -> bool;
  };
}

template <typename T>
concept bool
JustInequalityComparable() {
  return requires(T a, T b) {
    { a != b } -> bool;
  };
}

bool operator==(const JustInequalityComparable& a1,
                const JustInequalityComparable& a2) {
  return !(a1 != a2);
}

bool operator!=(const JustEqualityComparable& a1,
                const JustEqualityComparable& a2) {
  return !(a1 == a2);
}

template <typename T>
  requires JustEqualityComparable<T>()
bool useEq(T t1, T t2) {
  return t1 == t2;
}

struct A {
  int a;
};

bool operator==(const A& a1, const A& a2) {
  return a1.a == a2.a;
}

int main() {
  using namespace std;

  cout << boolalpha << useEq(A{4}, A{6}) << "\n";

  return 0;
}

===
~/bin/gcc/bin/g++ -std=c++1z -save-temps foo.cpp
g++: internal compiler error: Segmentation fault (program cc1plus)


Removing either the operator in line 19 or 24 makes the program compile.
Replacing the operator implemented with concepts in line 19 by a plain template
version makes the program compile as well.