Wsrtjtyk

Using C++, trying to implement: is_specialization_of

template<typename T, template<typename...> class Template>

struct is_specialization_of : std::false_type {};



template<template<typename...> class Template, typename... Tn>

struct is_specialization_of<Template<Tn...>, Template> : std::true_type {};



template<typename... Tn>

struct tstruct {};



template<typename... Tn>

using ustruct = tstruct<Tn...>;



int main( int argc, char **argv )

{

    printf( "test u<int> against u, return %sn", is_specialization_of<ustruct<int>, ustruct>::value ? "true" : "false" );

    printf( "test u<int> against t, return %sn", is_specialization_of<ustruct<int>, tstruct>::value ? "true" : "false" );

    printf( "test t<int> against u return %sn", is_specialization_of<tstruct<int>, ustruct>::value ? "true" : "false" );

    printf( "test t<int> against t, return %sn", is_specialization_of<tstruct<int>, tstruct>::value ? "true" : "false" );

    getchar();

    return 0;

}

Return:

test u<int> against u, return false

test u<int> against t, return true

test t<int> against u return false

test t<int> against t, return true

Looks like the type alias is not considered exactly as the original type

I am using Visual Studio Community 2017

Microsoft (R) C/C++ Optimizing Compiler Version 19.15.26732.1 for x64

However when trying to compile the same code using gcc, it returns:

test u<int> against u, return true

test u<int> against t, return true

test t<int> against u return true

test t<int> against t, return true

Is there anything I can do for a workaround?

Looks like the type alias is not considered exactly as the original type, or did I miss something?

The alias specialization is exactly the type it stands for. But the alias tempalte is an entirely distinct template. The reason for your output is specified by [temp.alias]

1 A template-declaration in which the declaration is an
alias-declaration declares the identifier to be an alias template. An
alias template is a name for a family of types. The name of the alias
template is a template-name.

2 When a template-id refers to the specialization of an alias
template, it is equivalent to the associated type obtained by
substitution of its template-arguments for the template-parameters in
the type-id of the alias template.

If we examine your test cases with the above two paragraphs in mind, we see that:

1. "test u<int> against u" - ustruct<int> is equivalent to specifying tstruct<int> directly. And tstruct<int> is not a specialization of ustruct. The trait needs to evaluate to false.




2. "test u<int> against t" - Here ustruct<int> is again equivalent to specifying tstruct<int> directly. And tstruct<int> is a specialization of tstruct. The trait should report true.




3. "test t<int> against u" - tstruct<int> is not a specialization of ustruct, like we observed previously. The trait should report false.




4. "test t<int> against t" - Should report true.

All your tests, when run in MSVC, conform to what the C++ standard says they should do. GCC is not conforming here, it's a compiler bug.