Looking at P2098r1:
The given implementation works fine for specialisations of 'vector', but not for specialisations of 'array', as follows:
#include <cstddef>
#include <array>
#include <type_traits>
#include <vector>
namespace std {
template< class T
, template<class...> class Primary >
struct
is_specialization_of : false_type {};
template< template<class...> class Primary
, class... Args >
struct
is_specialization_of< Primary<Args...>, Primary> : true_type {};
}
int main(void)
{
using std::is_specialization_of;
static_assert( is_specialization_of< std::vector<int>, std::vector >::value );
static_assert( is_specialization_of< std::array<int, 5>, std::array >::value );
}
Compiler output:
a.cpp: In function 'int main()':
a.cpp:22:73: error: type/value mismatch at argument 2 in template parameter list for 'template<class T, template<class ...> class Primary> struct std::is_specialization_of'
a.cpp:22:73: note: expected a template of type 'template<class ...> class Primary', got 'template<class _Tp, long unsigned int _Nm> struct std::array'
If 'is_specialization_of' is to be written into the Standard, shall it be worded that it requires compiler support to let it work with every template class (and not just the ones whose parameters are exclusively all types)? Or will the use of this feature be restricted to template classes whose parameters are all types?
I'm talking something like:
namespace std {
template< class T , /* COMPILER_SUPPORT */ Primary >
struct is_specialization_of : false_type {};
template< /* COMPILER_SUPPORT */ Primary , class... Args >
struct is_specialization_of< Primary<Args...>, Primary> : true_type {};
}