[pxx]

c++20 offers `consteval` to make this clear, but you can do some simple macro wizardry in c++11 to do this:

    #define foo(x) ( \
        (void)std::integral_constant<char, (x)[0]>::value, \
        foo_impl(x) \
    )

(the re-evaluation of x doesn't matter if it compiles). You can also use a user-defined literal which has a different ergonomic problem.