Missing case from "down with typename"

[ilya-biryukov]

Found by looking at g++.dg/cpp2a/typename16.C from the GCC test suite.
Clang produces an error here:

struct X { X(int); };
template<typename T> struct S {
  friend X::X(T::type);
  // <source>:3:13: error: qualified reference to 'X' is a constructor name rather than a type in this context
};

GCC and MSVC accept the code with no errors. See godbolt.

I believe Clang should accept this code without typename according to (temp.res)p4.3:

4. A qualified or unqualified name is said to be in a type-only context if it is the terminal name of
...
4.3 a decl-specifier of the decl-specifier-seq of a 
...
4.3.3 parameter-declaration in a member-declaration,125 unless that parameter-declaration appears in a default argument,

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[cor3ntin]

https://reviews.llvm.org/D152242