typedef changes meaning

Question

When I compile the following snippet with g++

template
class A
{};

template
class B
{
    public:
        typedef         


        

Answer 1

g++ is correct and conforming to the standard. From [3.3.7/1]:

A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.

Before the typedef, A referred to the ::A, however by using the typedef, you now make A refer to the typedef which is prohibited. However, since no diagnostic is required, clang is also standard conforming.

jogojapan's comment explains the reason for this rule. Take the following change to your code:

template
class A
{};

template
class B
{
    public:
        A a; // <-- What "A" is this referring to?
        typedef     A            A;
};

Because of how class scope works, A a; becomes ambiguous.