Using alias templates for sfinae: does the language allow it?

Question

I have just discovered the following technique. It looks very close to one of proposed concepts syntax, works perfectly on Clang, GCC and MSVC.

template 

        

Answer 1

It works and allowed because it relays on widely used C++ features allowed by the standard:

1. SFINAE in function parameters ([temp.over]/1, [temp.deduct]/6, [temp.deduct]/8):

   template 
   void foo(T&& v, typename std::enable_if::value>::type* = nullptr)
   { /* ... */ }
   

   _{we cannot deduce on the actual parameter like void foo(typename std::enable_if::value, T>::type&&) (CWG#549), but it is possible to workaround this limitation with template aliases (it is the trick I have presented in my question)}

2. SFINAE in template parameter declaration ([temp.deduct]/7):

   template ::value>::type* = nullptr>
   void foo(T&& v)
   { /* ... */ }
   

3. Alias templates in function parameters ([temp.alias]/2):

   template struct Alloc { /* ... */ };
   template using Vec = vector>;
   
   template
     void process(Vec& v)
     { /* ... */ }
   

4. Alias templates can have default parameters ([temp.param]/12, [temp.param]/15, [temp.param]/18)

5. Template parameters of alias templates parameterized with deducible types still can be deduced ([temp.deduct.type]/17):

I have accepted @Barry's answer and put this one (with concentrated info and about every aspect the trick uses) because a lot of people (including me) are scared of C++ standard voodoo language about template deduction stuff.