The below example is ill-formed - the requires clause should not be satisfied due to the failure to instantiate W1<int>, but GCC accepts the code nonetheless.

Clang rejects the code.

$ cat gcc-bug.cpp
template<class> concept C1 = true;

template<class T> using W1 = decltype(nonexistent<T>(0));

template<class T>
requires C1<W1<T>>
void func(T);

void test() {
  func(0);
}

$ g++ -c -std=c++20 gcc-bug.cpp

$ clang++ -c -std=c++20 gcc-bug.cpp
gcc-bug.cpp:10:3: error: no matching function for call to 'func'
   10 |   func(0);
      |   ^~~~
gcc-bug.cpp:7:6: note: candidate template ignored: constraints not satisfied [with T = int]
    7 | void func(T);
      |      ^
gcc-bug.cpp:3:39: note: because substituted constraint expression is ill-formed: use of undeclared identifier 'nonexistent'
    3 | template<class T> using W1 = decltype(nonexistent<T>(0));
      |                                       ^
1 error generated.