Clang accepts incorrect template decl

[wangbo15]

The following code is accepted by clang-trunk, while GCC and MSVC reject it.

template <typename>
concept a = b<[] {}>();
int main() {}

Please refer to: https://godbolt.org/z/hvr3xn5xd

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Bo Wang (wangbo15)

The following code is accepted by clang-trunk, while GCC and MSVC reject it.

template <typename>
concept a = b<[] {}>();
int main() {}

Please refer to: https://godbolt.org/z/hvr3xn5xd

[cor3ntin]

It seems unrelated to concepts

template <typename> bool b = watman<[] {}>(); // accepts
template <typename> bool a = wat<0>(); // rejects

https://godbolt.org/z/Y9GT6v8a9

[cor3ntin]

I think it's related to https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0846r0.html - still trying to figure the set of cases we could/should diagnose