passing a static constexpr variable by universal reference?

Question

In the following, static constexpr member L is initialized in-class A and then passed by value or by (universal) reference. The latter

Answer 1

You need to define A::L outside its class in a source file

constexpr size_t A::L;

Live example using Clang

For header-only code, and if your class A is not already a template, you can define a class template A_ with a void default value, and write a typedef for A in terms of that

template
struct A_
{
    static constexpr size_t L = 4;

    template 
    void member_ref(T&& x) { cout << std::forward(x) << endl; }

    template 
    void member_val(T x) { cout << x << endl; }

};

template
constexpr size_t A_::L;

using A = A_<>;

Live Example.

NOTE: this business can involve a fair amount of boiler-plate. It is good to note that one can write

template
<
    class MyConcept1, 
    class MyConcept2, 
    class YetAnotherConcept
    // more long and well-documented template parameter names
>
struct A
{
    // many static constexpr variabels
};

template
constexpr SomeType A::some_var;

// many more one-liners.

Template parameters just have formal names, they don't have to be the same everywhere (just put them in the right order everywhere, though!).