alias template not recognized by is_same

[Oliver Rosten]

template<class T> struct component {};

template<class T> using alias = component<T>;

template<class T, class C> struct thing {};

      

template<class T, template<class> class C> struct thing2 {};   

static_assert(std::is_same_v<thing<int, component<int>>, thing<int, alias<int>>>); 

// succeeds

static_assert(std::is_same_v<thing2<int, component>, thing2<int, alias>>); 

// fails on both gcc and clang

I can't think of any good reason why the second assert should fail, but perhaps I've missed something!

[Todd Fleming]

alias<T> and component<T> are the same type. However, alias and component are separate templates. That makes thing2<int, component> and thing2<int, alias> separate types.

Todd

[Christof Meerwald]

On Tue, Dec 05, 2017 at 10:21:09PM +0000, 'Oliver Rosten' via ISO C++ Standard - Discussion wrote:
> template<class T> struct component {};
>
> template<class T> using alias = component<T>;

see CWG issue 1286
https://wg21.cmeerw.net/cwg/issue1286


Christof

--

http://cmeerw.org sip:cmeerw at cmeerw.org
mailto:cmeerw at cmeerw.org xmpp:cmeerw at cmeerw.org