Unqualified lookup for `friend elaborated-type-specifier` in local classes escapes the nearest enclosing block scope

[ckwastra]

Test code (CE):

struct A {
  template <class T> using type = typename T::type;
};
void f() {
  class B {
    friend class A;
    using type = void;
  };
  struct A {
    // Clang/MSVC: error ❌
    //    GCC/EDG: OK    ✅
    using type = B::type;
  };
  // Clang/MSVC: OK    ❌
  //    GCC/EDG: error ✅
  using type = ::A::type<B>;
}

According to [dcl.type.elab]/4:

Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the nearest enclosing namespace or block scope; [...]

Therefore, in the declaration friend class A above, ::A (in global scope) should not be found. The friendship of B should instead be granted to the local class A declared inside f.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Vincent X (ckwastra)

**Test code ([CE](https://godbolt.org/z/4E7vP6vKj)):**

struct A {
  template <class T> using type = typename T::type;
};
void f() {
  class B {
    friend class A;
    using type = void;
  };
  struct A {
    // Clang/MSVC: error ❌
    //    GCC/EDG: OK    ✅
    using type = B::type;
  };
  // Clang/MSVC: OK    ❌
  //    GCC/EDG: error ✅
  using type = ::A::type<B>;
}

According to [[dcl.type.elab]/4](https://eel.is/c++draft/dcl.type.elab#4):

> Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the nearest enclosing namespace or block scope; [...]

Therefore, in the declaration friend class A above, ::A (in global scope) should not be found. The friendship of B should instead be granted to the local class A declared inside f.

[keinflue]

That's CWG 2634 which doesn't seem implemented yet. (Although pre-C++23 wording was clear in specifying the same behavior as noted in the issue linked below.)