Bug 66879

Clang compiles it OK, but EDG says:

"a.cc", line 5: error: class "A" cannot be defined in the current scope
      class A { void f() {} };

This seems similar to the G++ error, which is due to the name A referring to ::A, which should be defined in the global scope.

I'm not sure whether the definition of Y::A should be an error, or (as Clang thinks) should declare a new type, which cannot be referred to due to the ambiguity caused by the using-declaration.

(In reply to Anders Granlund from comment #0)
> Note that if we remove the member function definition in the given program
> it compiles without errors.

I think that's the real bug. G++ should not accept this program:

class A;

namespace Y {
    using ::A;
    class A { };
}

int main()
{
  Y::A a;
  ::A* p = &a;
}

Clang rejects it because Y::A in main() is ambiguous, but EDG rejects it with the same error as for the original testcase:

"a.cc", line 5: error: class "A" cannot be defined in the current scope
      class A { };
            ^

I believe EDG is right to reject that class-specifier because the class name A has already been introduced into that scope, and it cannot be a definition of ::A because it doesn't use a nested-name-specifier and it isn't in the namespace (or an enclosing namespace) of ::A.

Thanks for the comments! Now I remember the following bug report that I sent to clang:

https://llvm.org/bugs/show_bug.cgi?id=24030

That bug has now been confirmed and fixed in clang, so yes this is the actual bug and it should be fixed in gcc also.

So confirming as accepts-invalid with this testcase:

class A;

namespace Y {
    using ::A;
    class A { };
}

*** Bug 66888 has been marked as a duplicate of this bug. ***

*** Bug 66889 has been marked as a duplicate of this bug. ***

(In reply to Jonathan Wakely from comment #4)
> So confirming as accepts-invalid with this testcase:
> 
> class A;
> 
> namespace Y {
>     using ::A;
>     class A { };
> }

Here are two similar test cases pulled in from my related bug reports (i have marked them as resolved duplicates).

The following ill-formed programs are also accepted by gcc:

  namespace R { struct f; }
  namespace S { using R::f; }
  struct S::f {};
  int main() {} 

  The above is ill-formed by [dcl.meaning]p1 ( http://eel.is/c++draft/dcl.meaning#1 )

  namespace P { struct S; }
  namespace R { using namespace P; }
  struct R::S {};
  int main() {}

  The above is also ill-formed by [dcl.meaning]p1

In general redeclarations must be done directly and not indirectly.

*** Bug 66901 has been marked as a duplicate of this bug. ***

*** Bug 66878 has been marked as a duplicate of this bug. ***

Here comes two more test cases moved in from my related ICE on compile bug reports:

namespace X {
    struct S;
    namespace Y {
        namespace Z = X;
        struct Z::S {};
    }
}
int main() {}

struct S;
namespace H {
    namespace P { using ::S; }
    struct P::S {};
}
int main() {}

The above programs bot gives ICE on compile (infinite recursion in push_inner_scope_r -> stack overflow). They also involve trying to perform redeclarations indirectly.