[Bug c++/100687] New: [modules, concepts] imported concept gives different result

[webrown.cpp at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100687

            Bug ID: 100687
           Summary: [modules, concepts] imported concept gives different
                    result
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: webrown.cpp at gmail dot com
  Target Milestone: ---

Using recent trunk [g++-HEAD (Homebrew GCC HEAD-5380e3c) 12.0.0 20210513],
compiling with significant flags -std=c++23 -fmodules-ts -c.

I have two source files, as follows:

//////////////////////
export module bug_a;

template< class T >
inline constexpr bool probe = false;
//
template< class R, class C >
inline constexpr bool probe<R C::* > = true;

export
template< class T >
concept mbr_ptr = probe<T>;

struct S { int f( ); };
using mf_t = decltype( &S::f );

static_assert( mbr_ptr< mf_t > );
//////////////////////

This bug_a module compiles successfully.  In particular, the static_assert
passes.

//////////////////////
export module bug_b;

import bug_a;

struct S { int f( ); };
using mf_t = decltype( &S::f );

static_assert( mbr_ptr< mf_t > );
//////////////////////

This bug_b module does not compile.  The diagnostics are reproduced below,
including the somewhat mysterious lone apostrophe:

bug_b.cc:8:16: error: static assertion failed
    8 | static_assert( mbr_ptr< mf_t > );
      |                ^~~~~~~~~~~~~~~
bug_b.cc:8:16: note: constraints not satisfied
'
bug_a.cc:12: confused by earlier errors, bailing out

Please note that the static_assert and supporting declarations in module bug_a
are identical to those in module bug_a, yet one compiles while the other does
not.

Thus we seem to have two problems:  (1) the lone apostrophe amongst the
diagnostics, and (2) the inconsistent evaluation of identical expressions.

Finally, please note that if I use a class template (instead of the variable
template) in module bug_a, both modules' static_asserts now compile without
complaint:

//////////////////////
template< class T >
struct probe { static constexpr bool value = false; };
//
template< class R, class C >
struct probe<R C::*> { static constexpr bool value = true; };

export
template< class T >
concept
  mbr_ptr = probe<T>::value;
//////////////////////