What is template<> template<> syntax in the context of template-alias for?

Question

(This question is not about template template arguments.)

I just discovered that GCC compiles such code

template <typename A, typename B>
struct P {};

template <typename A>
template <typename B>
using Q = P<A, B>;

where Q is a doubly-templated name.

But I can't use this. When I write Q<short><long>, I get

template_template.cpp:10:5: error: ‘Q<short int>’ is not a template
     Q<short><long>{};
     ^~~~~~~~
template_template.cpp:10:20: error: invalid use of incomplete type ‘Q<short int>’
     Q<short><long>{};
                    ^
template_template.cpp:2:8: note: declaration of ‘Q<short int>’
 struct P {};

Why is the first snippet compiled?

Is there a syntax to convince the compiler that Q<short> is actually a template?

// GCC 6.3.0

Answer 1

The C++14 standard says in 14p1:

The declaration in a template-declaration shall
— declare or define a function, a class, or a variable, or
— define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or
— define a member template of a class or class template, or
— be an alias-declaration

Here the declaration within the template-declaration is none of the above (it is another template-declaration, which itself contains an alias-declaration), and therefore the code is invalid.

The relevant parts of the grammar are:

template-declaration:
    template <template-parameter-list> declaration

alias-declaration:
    using identifier attribute-specifier-seq_opt = type-id ;

where a declaration can be a template-declaration, an alias-declaration, or other types of declarations.

Note that the grammar itself accepts the given code, but the additional restrictions in the text above make it invalid.