Compiler accepting ill-formed program trying to define a struct via `using-declaration`

[ebdd6ebc-1612-4e9b-aee1-f7d2b6361d5f]

Bugzilla Link	24138
Version	trunk
OS	All
CC	@yeah-boi,@majnemer,@DougGregor,@zygoloid

Extended Description

Consider the following program:

  namespace R {
      struct f;
  }

  namespace S {
      using R::f;
  }
  struct S::f {};

  int main() {}

But clang compiles it without any error messages. The expected behaviour is to get an error message.

The program is ill-formed by the following clauses in the c++ standard:

• [class]p11 ( http://eel.is/c++draft/class#11 ): Note especially the sentence "i.e., not merely inherited or introduced by a using-declaration"

• [dcl.meaning]p1 ( http://eel.is/c++draft/dcl.meaning#1 ): Not especially the sentence "the member shall not merely have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id."

[ebdd6ebc-1612-4e9b-aee1-f7d2b6361d5f]

This should also be ill-formed, but clang accepts it too (it an even simpler test-case):

  struct S;
  namespace N { using ::S; }
  struct N::S {};

[ebdd6ebc-1612-4e9b-aee1-f7d2b6361d5f]

Disregard the previous comment. It was indented for the same bug in GCC.

[ebdd6ebc-1612-4e9b-aee1-f7d2b6361d5f]

This bug seems to be more general than struct definitions. It also exists for variable declarations like this:

  namespace X { extern int i; }

  namespace N { using X::i; }

  int N::i = 1;

  int main() {}

The above program is ill-formed by [dcl.meaning]p1, but no error message is given.