[Bug c++/91662] New: [concepts] unable to deduce placeholder type, should be accepted

[frederik.engels24 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91662

            Bug ID: 91662
           Summary: [concepts] unable to deduce placeholder type, should
                    be accepted
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frederik.engels24 at gmail dot com
  Target Milestone: ---

The following should compile but doesn't on current gcc-trunk (20190902)
with options: "-std=c++2a -fconcepts"

godbolt: https://godbolt.org/z/3jvFqX

#include <type_traits>
#include <utility>

template<typename T, typename U>
concept same_as = std::is_same_v<T, U> && std::is_same_v<U, T>;

template<typename LHS, typename RHS>
concept assignable_from = 
    requires(LHS lhs, RHS&& rhs) {
        { lhs = std::forward<RHS>(rhs) } -> same_as<LHS>;
    };

template<assignable_from<int> T>
void foo(T&& t) {}

int main()
{
    int i = 0;
    foo(i);
}

with output

<source>: In function 'int main()':

<source>:19:10: error: cannot call function 'void foo(T&&) [with T = int&]'

   19 |     foo(i);

      |          ^

<source>:14:6: note:   constraints not satisfied

   14 | void foo(T&& t) {}

      |      ^~~

<source>:8:9: note: within 'template<class LHS, class RHS> concept const bool
assignable_from<LHS, RHS> [with LHS = int&; RHS = int]'

    8 | concept assignable_from =

      |         ^~~~~~~~~~~~~~~

<source>:8:9: note:     with 'int& lhs'

<source>:8:9: note:     with 'int&& rhs'

<source>:8:9: note: unable to deduce placeholder type 'same_as<int&>' from 'lhs
=(forward<int>)(rhs)'

ASM generation compiler returned: 1

<source>: In function 'int main()':

<source>:19:10: error: cannot call function 'void foo(T&&) [with T = int&]'

   19 |     foo(i);

      |          ^

<source>:14:6: note:   constraints not satisfied

   14 | void foo(T&& t) {}

      |      ^~~

<source>:8:9: note: within 'template<class LHS, class RHS> concept const bool
assignable_from<LHS, RHS> [with LHS = int&; RHS = int]'

    8 | concept assignable_from =

      |         ^~~~~~~~~~~~~~~

<source>:8:9: note:     with 'int& lhs'

<source>:8:9: note:     with 'int&& rhs'

<source>:8:9: note: unable to deduce placeholder type 'same_as<int&>' from 'lhs
=(forward<int>)(rhs)'

Execution build compiler returned: 1