`decltype`ing static data member template of class template specified with `auto` won't be resolved

[yaito3014]

Following valid C++ code won't compile:

template<class>
struct S {
  template<class>
  inline static auto mem = 42;
};

using T = decltype(S<void>::mem<void>);

T y = 42;

int main() {}

Compiling with:

clang++-22 -std=c++17 source.cpp

Errors are as follows:

source.cpp:9:3: error: cannot initialize a variable of type 'T' (aka 'auto') with an rvalue of type 'int'
    9 | T y = 42;
      |   ^   ~~
1 error generated.

This may be related to #56652, since it appears that the Clang frontend does not resolve the auto type, leaving code generation to handle an undeduced type.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: KakeyamaY (yaito3014)

Following valid C++ code won't compile: ```cpp template<class> struct S { template<class> inline static auto mem = 42; };

using T = decltype(S<void>::mem<void>);

T y = 42;

int main() {}

Compiling with:

clang++-22 -std=c++17 source.cpp

Errors are as follows:

source.cpp:9:3: error: cannot initialize a variable of type 'T' (aka 'auto') with an rvalue of type 'int'
9 | T y = 42;
| ^ ~~
1 error generated.

This may be related to #<!-- -->56652, since it appears that the Clang frontend does not resolve the auto type, leaving code generation to handle an undeduced type.
</details>

[efriedma-quic]

It looks like the issue is just that we're not instantiating the definition. If you force instantiation by odr-using the variable, it works.

[cor3ntin]

Without initialization:

template <class> struct S {
    template <class>
    inline static auto mem = [] {
        static_assert(false);
        return 42;
    }();
};
using T = decltype(S<void>::mem<void>);

https://godbolt.org/z/vGcqh3Po5