c++ - Why are redundant scope qualifications supported by the compiler, and is it legal?

Question

I tested on two compilers, and was surprised to see both support the following definition without complaint:

class A {
  A();
};

A::A::A() {}
   ^^^

Note that this also succeeds for methods, although it is flagged when the declaration is over-qualified.

Questions:

• Is this a valid C++ program?
• If so, what purpose does it serve - or is it merely a byproduct?

Updated Detail:

In case the original question was not clear or too short: I'm curious why redundant qualifications are permitted on the definition (emphasis also added above).

---

Clang an Apple's GCC 4.2 + LLVM were the compilers

Answer 1

Yes, it's allowed (§9/2):

The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name. For purposes of access checking, the injected-class-name is treated as if it were a public member name.

For information about the reasoning that lead to class name inject, you might want to read N0444.