Clang ICE on specialized template taking a member function pointer

[tambry]

The following crashes on trunk, but not on 20 or earlier.

template<typename T, T>
struct proxy;

template<typename T, typename R, typename... Args, R(T::*mf)()>
struct proxy<R(T::*)(Args...), mf>;

I was just trying to write some code to get the return of a member function. 🥲

Godbolt
GCC bug

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Raul Tambre (tambry)

The following crashes on trunk, but not on 20 or earlier. ``` template<typename T, T> struct proxy;

template<typename T, typename R, typename... Args, R(T::mf)()>
struct proxy<R(T::)(Args...), mf>;

[Godbolt](https://godbo.lt/z/8qcq1vshn)
</details>

[zyn0217]

@mizvekov

[mizvekov]

There is a direct issue here that's pretty easy to fix, unfortunately this then hits on some pretty complicated unresolved issues regarding partial ordering of templates with placeholder types.

[tambry]

Seems to have been fixed. Can't be bothered to bisect for the commit.