[Bug c++/97988] New: [C++20] Forward-declared class type declared inside requires-expression gives weird inconsistencies

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97988

            Bug ID: 97988
           Summary: [C++20] Forward-declared class type declared inside
                    requires-expression gives weird inconsistencies
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/sxWY1f
template<class P>
concept C = requires (P ptr) { (struct D*)ptr; };

struct D {};
D d;

====

Clang accepts. GCC rejects; GCC's error messages imply that GCC is sometimes
treating `D` as a class template `template<class P> struct D` and sometimes
treating it as a normal class type, as if its internal representation is
inconsistent.

<source>:6:3: error: class template argument deduction failed:
    6 | D d;
      |   ^
<source>:6:3: error: no matching function for call to 'D()'
<source>:3:40: note: candidate: 'template<class P> D()-> D'
    3 | concept C = requires (P ptr) { (struct D*)ptr; };
      |                                        ^
<source>:3:40: note:   template argument deduction/substitution failed:
<source>:6:3: note:   couldn't deduce template parameter 'P'
    6 | D d;
      |   ^
<source>:3:40: note: candidate: 'template<class P> D(D)-> D'
    3 | concept C = requires (P ptr) { (struct D*)ptr; };
      |                                        ^
<source>:3:40: note:   template argument deduction/substitution failed:
<source>:6:3: note:   candidate expects 1 argument, 0 provided
    6 | D d;
      |   ^

====

I can also make GCC segfault by trying to use `D` as an NTTP, but I think
that's essentially just a duplicate of #95159 #95291 #96123 #97749 etc., not
necessarily related to the rejects-invalid bug described here.

template<class P>
concept C = requires (P ptr) { (struct D*)ptr; };

struct D { constexpr D(); };
template<D d> struct S {};
S<1> s;