Shafik Yaghmour

B is invalid see [class.qual]p2:

In a lookup in which function names are not ignored28 and the nested-name-specifier nominates a class C:

• if the name specified after the nested-name-specifier, when looked up in C, is the injected-class-name of C ([class]), or

• in a using-declarator of a using-declaration ([namespace.udecl]) that is a member-declaration, if the name specified after the nested-name-specifier is the same as the identifier or the simple-template-id’s template-name in the last component of the nested-name-specifier,

the name is instead considered to name the constructor of class C. [ Note: For example, the constructor is not an acceptable lookup result in an elaborated-type-specifier so the constructor would not be used in place of the injected-class-name.— end note] Such a constructor name shall be used only in the declarator-id of a declaration that names a constructor or in a using-declaration. [ Example:

 struct A { A(); };
 struct B: public A { B(); };

 A::A() { }
 B::B() { }

 B::A ba;            // object of type A
 A::A a;             // error, A​::​A is not a type name
 struct A::A a2;     // object of type A

— end example]

The class name is injected into the scope of the class itself but depending on the context the nested-name-specifier refers to the constructor not the type

Also see Defect Report 147