[Bug c++/126943] New: "template argument 1 is invalid" error on using function parameter in type requirement

[cmingyi01 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126943

            Bug ID: 126943
           Summary: "template argument 1 is invalid" error on using
                    function parameter in type requirement
           Product: gcc
           Version: 16.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cmingyi01 at gmail dot com
  Target Milestone: ---

The following valid C++20 code is rejected by GCC:
```
template <int> class C;
constexpr int g(auto const&) { return 0; }

template <class T>
int f(T a)
requires requires { typename C<g(a)>; }
{ return 0; }

int i = f(0);
```

Output:
```
<source>:6:36: error: template argument 1 is invalid
    6 | requires requires { typename C<g(a)>; }
      |                                    ^
<source>:6:36: error: template argument 1 is invalid
<source>:6:36: error: template argument 1 is invalid
<source>:6:36: error: template argument 1 is invalid
<source>:6:36: error: template argument 1 is invalid
<source>:6:36: error: template argument 1 is invalid
<source>:6:36: error: template argument 1 is invalid
<source>:6:30: error: 'C' is not a type
    6 | requires requires { typename C<g(a)>; }
      |                              ^
<source>:9:10: error: no matching function for call to 'f(int)'
    9 | int i = f(0);
      |         ~^~~
  • there is 1 candidate
    • candidate 1: 'template<class T> int f(T) requires 
<erroneous-expression>'
      <source>:5:5:
          5 | int f(T a)
            |     ^
      • template argument deduction/substitution failed:
        • constraints not satisfied
```

See https://compiler-explorer.com/z/jdczc8Kz6.