#include <type_traits>
template< typename T > struct Witness { };

struct A {
    template< typename T >
    A( Witness< T > ) { /* ... */ }
};

template< int val >
int getX() { return val; }

template< int val >
int getY( std::integral_constant< int, val > ) {
    return val;
}
template< int val >
int getZ( std::integral_constant< int, val > singleton ) {
    return getY( singleton );
}
int main( void ) {
    A a{ Witness< long >() }; // new ctor call syntax needed
    getX< 1 >();
    return getZ( std::integral_constant< int, 5 >() );
}