Is it possible to check if a user literal is defined for given type and argument?

Question

I want to check at compile-time if user literal _name is defined for type Ret and argument Arg. While I have half-solution, it require

Answer 1

With is_detected functions family, you may just do

template 
using has_literal_x_type = decltype(operator"" _x(std::declval()));

template 
using has_literal_x = std::is_same>;

And test it with

static_assert(!has_literal_x::value, "unexpected");
static_assert(!has_literal_x::value, "unexpected");
static_assert(!has_literal_x::value, "unexpected");
static_assert(!has_literal_x::value, "unexpected");
static_assert(!has_literal_x::value, "unexpected");

Demo