Identity of template aliases

Question

Let's consider a set of template aliases:

template<class T> using foo = T*;
template<class T> using bar = T*;
template<class T> using buz = foo<T>;

template< template<class>class TT > struct id {};

using id_foo = id<foo>;
using id_bar = id<bar>;
using id_buz = id<buz>;

Are id_foo, id_bar, id_buz same or different types? Are foo, bar, buz same or different templates?

Various compilers have different opinions on that. Particularly,

MSVC 2015 and clang 3.5 treat they all are different
gcc 4.9 treats buz is same as foo

Standard C++11 in the chapter 14.5.7 "Alias templates" is unclear.

[cppreference](http://en.cppreference.com/w/cpp/language/type_alias)'s explanation seems to imply that yes, they are the same IMO. Edit: Relevant snippet: `An alias template is a template which, when specialized, is equivalent to the result of substituting the template arguments of the alias template for the template parameters in the type-id` — AndyG, Mar 16 '15 at 15:31
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1286 — T.C., Mar 16 '15 at 16:03
Answers to the following questions may be helpful: [_Using a template alias instead of a template within a template_](http://stackoverflow.com/q/17392621/3043539) and [_Strange behaviour of is_same_template on template aliases_](http://stackoverflow.com/q/22892052/3043539). — Constructor, Mar 17 '15 at 05:27
@T.C. Other than it being only a link, how is that not an answer? :p — Yakk - Adam Nevraumont, Mar 17 '15 at 13:30

score 1 · Answer 1 · edited May 23 '17 at 12:06

As T.C. pointed out in his comment to the question, this is a known hole of the standard.

The current wording of 14.5.7 [temp.alias] deals only with the equivalence of a specialization of an alias template with the type-id after substitution. Wording needs to be added specifying under what circumstances an alias template itself is equivalent to a class template.

That is: any specialization of foo, bar and buz will represent the same type. But there is no guarantee that, when used as a template parameter, foo, bar and buz are interchangeable.

Identity of template aliases

1 Answers1