[Bug c++/69364] New: [concepts] failure to properly order constraints when using fold expressions

[ryan.burn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69364

            Bug ID: 69364
           Summary: [concepts] failure to properly order constraints when
                    using fold expressions
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ryan.burn at gmail dot com
  Target Milestone: ---

The below code produces an ambiguous overload error.

I'm not completely sure if the concept-lite standard dictates that it should
compile (see discussion:
http://stackoverflow.com/questions/34843745/how-are-fold-expressions-used-in-the-partial-ordering-of-constraints),
but it seems like it ought to work.

///////////////////////////////////////////////////////////////////////
#include <iostream>
#include <type_traits>

template <class T> concept bool A = std::is_move_constructible<T>::value;
template <class T> concept bool B = std::is_copy_constructible<T>::value;

template <class T> concept bool C = A<T> && B<T>;

template <class _a, class _b>
  requires A<_a> && A<_b>
void f(_a a, _b b) {
  std::cout << "a\n";
}

template <class _a, class _b>
  requires C<_a> && C<_b>
void f(_a a, _b b) {
  std::cout << "c\n";
}

template <class... _tx>
  requires (A<_tx> && ...)
void g(_tx... tx) {
  std::cout << "a\n";
}

template <class... _tx>
  requires (C<_tx> && ...)
void g(_tx... tx) {
  std::cout << "c\n";
}

int main() {
  f(3, 7.0); // works with no fold expressions
  g(3, 7.0); // ambiguous overload error

  return 0;
}
///////////////////////////////////////////////////////////////////////