const dependent names returned from template functions, where does const go?

Question

Suppose that I have a template function (e.g., foo), that returns a const dependent type. The options to qualify the return type as const is to either put const at the left of typename keyword:

template<typename T>
const typename T::bar
^^^^^
foo(T const& baz) {
  ...
}

or at the right of the dependent type:

template<typename T>
typename T::bar const
                ^^^^^
foo(T const& baz) {
  ...
}

But what if I put the const qualifier between the typename keyword and the dependent type?

template<typename T>
typename const T::bar
         ^^^^^
foo(T const& baz) {
  ...
}

The above as expected, fails to compile for GCC and CLANG, but to my surprise VC++ compiles it fine.

Q:

• Is this a VC++ extension?
• Does the C++ standard say anything about where is the appropriate place to put const qualifier in such a context?

Answer 1

Does the C++ standard says anything about where is the appropriate place to put const qualifier in such a context?

Yes. typename appears in a typename-specifier, whose production is

typename nested-name-specifier identifier
typename nested-name-specifier template_optsimple-template-id

In other words, it must be followed directly by a nested-name-specifier. const is not allowed.