typedef changes meaning

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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 T>
class A
{};

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

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

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