CWG2974 [temp.deduct.type] Missing non-deduced context for nested-name-specifier of qualified-id of a template

[hubert-reinterpretcast]

Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): temp.deduct.type

Link to reflector thread (if any): N/A

Issue description:
https://wg21.link/temp.deduct.type#5.1 has, as a non-deduced context,

• The nested-name-specifier of a type that was specified using a qualified-id.

This does not cover templates named using a qualified-id. There is implementation divergence:

template <template <typename> class>
struct B;

struct C {
  template <typename>
  struct Nested;
};

template <typename T>
void f(T *, B<T::template Nested> *);

void g(C *cp) {
  f(cp, 0);  // Clang rejects; `T::` in the declaration of `f` is not a non-deduced context
}

Online compiler link: https://godbolt.org/z/8sTvW7W94

Suggested resolution:
Modify https://wg21.link/temp.deduct.type#5.1:

• The nested-name-specifier of a type or template that was specified using a qualified-id.

[frederick-vs-ja]

Actually, Clang can't deduce T from B<T::template Nested>* either (demo). So I think all investigated implementations agree with the proposed wording, and the apparent divergence is just due to a bug of Clang.

[jensmaurer]

CWG2974