When and how to use a template literal operator?

On cppreference there is a mentioning that one can have templated user-literal operators, with some restrictions:

If the literal operator is a template, it must have an empty parameter list and can have only one template parameter, which must be a non-type template parameter pack with element type char, such as

template <char...> double operator "" _x();

So I wrote one like in the code below:

template <char...> 
double operator "" _x()
{
    return .42;
}

int main()
{
    10_x; // empty template list, how to specify non-empty template parameters?
}

Question:

1. The code works, but how can I use the operator with some non-empty template parameters? 10_x<'a'>; or 10_<'a'>x; does not compile.
2. Do you have any example of real-world usage of such templated operators?