clang++ incorrectly accepts "typename" in "using-declaration" without a nested-name-specifier

[llvmbot]

Bugzilla Link	15362
Version	3.1
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

According to the C++11 standard, the production of the rule using-declaration is:

using typename[opt] nested-name-specifier unqualified-id ; using :: unqualified-id ;

But it seems that clang (3.1 at least) accepts also this syntax:

using typename :: unqualified-id ;

Which is not part of the standard.

The following little code illustrates this:

typedef int T;
namespace foo {
  int T;
  namespace bar {
    using typename ::T; // < this should be rejected because of the presence of 'typename'
    T a;
  }
}

[AaronBallman]

This issue happens as of Clang 16.0.0: https://godbolt.org/z/n79z1Kqvd

[llvmbot]

@llvm/issue-subscribers-c-11

[zwuis]

This code should be well-formed. See CWG1887.