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(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
885
Templates in c++ is defined as a blueprint or formula for creating a generic class or a function. To simply put, you can create a single function or single class to work with different data types using templates. C++ template is also known as generic functions or classes which is a very powerful feature in C++.
Type alias is a name that refers to a previously defined type (similar to typedef). Alias template is a name that refers to a family of types.
In C++ this can be achieved using template parameters. A template parameter is a special kind of parameter that can be used to pass a type as argument: just like regular function parameters can be used to pass values to a function, template parameters allow to pass also types to a function.
In C++11, a type alias is a name for another already declared type, and an alias template is a name for another already declared template. Both of these types of aliases are introduced with a new using syntax.
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-seqopt = 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.
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