[concepts] out-of-line definition of constrained members is not accepted

[Ivan171]

Bugzilla Link	50276
Version	trunk
OS	Windows NT
CC	@Ivan171,@zygoloid

Extended Description

$ cat utf.cpp

#include <concepts>

template <typename>
struct utf_t;

template <typename T>
    requires (std::is_same_v<T, char32_t>) ||
             (std::is_same_v<T, wchar_t> && sizeof(wchar_t) == sizeof(char32_t))
struct utf_t<T> {
  utf_t() {
  }

  template <typename Input>
  static auto length(Input begin, Input const end) -> unsigned;
};

template <typename T>
    requires (std::is_same_v<T, char32_t>) ||
             (std::is_same_v<T, wchar_t> && sizeof(wchar_t) == sizeof(char32_t))
template <typename Input>
inline auto utf_t<T>::length(Input begin, Input const end) -> unsigned {
  return 0;
}

$ clang++ -std=c++20 utf.cpp

utf.cpp:21:23: error: out-of-line definition of 'length' from class 'utf_t<type-parameter-0-0>' without definition
inline auto utf_t<T>::length(Input begin, Input const end) -> unsigned {
            ~~~~~~~~~~^
1 error generated.

No matter how you constrain it, it doesn't work: https://godbolt.org/z/qqaM89nTx

GCC compiles it without any error: https://godbolt.org/z/PM3seoYvW

[palchak-google]

Here's a slightly simpler example that also demonstrates how easy it is to hit this defect:
https://godbolt.org/z/55T5vvK3f

[usx95]

Simplified more (any concept works and member does not have to be static.)

template<typename T>
struct S;

template <typename>
concept True = true;

template<True I>
struct S<I> {
    void value();
};

template<True I>
void S<I>::value() {};

https://godbolt.org/z/TTv4hx141

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[jwakely]

Another example from #58124, this one reduced from libstdc++ code:

template<class T> concept C1 = true;
template<class T> concept C2 = C1<T> && true;

template<C1> struct X { };
template<C2 T> struct X<T> { struct Y; };

template<C2 T> struct X<T>::Y { }; // (1)