[clang] Wrong class specialization is selected when alias template is used for template template parameter

[carljohnsonnewhampshire-hue]

The second static assert in the following trivial example code fails due to incorrect class specialization being selected:

template <template <typename...> class, class>
struct match_class_specialization
{
    enum { matches = 0 };
};

template <template <typename...> class Type, typename... Ts>
struct match_class_specialization<Type, Type<Ts...>>
{
    enum { matches = 1 };
};



template <typename...> struct my_type {};

static_assert(match_class_specialization<my_type, my_type<int, int>>{}.matches);



template <typename... Ts> using my_type_alias  = my_type<Ts...>;

static_assert(match_class_specialization<my_type_alias, my_type<int, int>>{}.matches);


int main() {}

This compiles fine in GCC.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (carljohnsonnewhampshire-hue)

The second static assert in the following trivial example code fails due to incorrect class specialization being selected: ```C++ template <template <typename...> class, class> struct match_class_specialization { enum { matches = 0 }; };

template <template <typename...> class Type, typename... Ts>
struct match_class_specialization<Type, Type<Ts...>>
{
enum { matches = 1 };
};

template <typename...> struct my_type {};

static_assert(match_class_specialization<my_type, my_type<int, int>>{}.matches);

template <typename... Ts> using my_type_alias = my_type<Ts...>;

static_assert(match_class_specialization<my_type_alias, my_type<int, int>>{}.matches);

int main() {}

This compiles fine in GCC. 
</details>

[shafik]

clang/MSVC reject gcc/edg accept: https://godbolt.org/z/WsKxaorKc

CC @mizvekov @hubert-reinterpretcast

[mizvekov]

Looks like CWG1286, which GCC and EDG implements the proposed resolution, but we don't.

[hubert-reinterpretcast]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92060