c++ - Why don't default-template arguments work on using declarations? -

i'm trying search alternative following dilemma. know how when have template class/function default template-argument have apply angle brackets when they're empty? attempt @ making fix. know can use simple typedef (typedef x<> l) don't want use different names refer class.

so tried following. reason when supply type template argument still doesn't work. why that?

#include <type_traits>  template <typename = void> struct x {};  template <typename t = void> using l = typename std::conditional<               std::is_void<t>::value,               x<>,               x<t> >::type;  int main() {     l l; } 

errors:

prog.cpp: in function ‘int main()’:
prog.cpp:10:7: error: missing template arguments before ‘l’
prog.cpp:10:7: error: expected ‘;’ before ‘l’

the syntax same other type templates: need provide empty template brackets default templates:

l<> l; 

the using declaration redundant since conditional inside nothing – you remove it, yielding template <typename t = void> using l = x<t>; – clearly not want.

and here’s thing: there no way around this; type templates distinct types (for reasons, too!) , cannot treat latter former – you have instantiate template type.