<ranges>: Poor error message when ranges::to cannot find a way to create the result

[cpplearner]

Currently, if ranges::to cannot create the result, the error message just says "the program is ill-formed per N4950 [range.utility.conv.to]/2.3".

STL/stl/inc/ranges

Line 10377 in be81252

static_assert(_Always_false<_Container>, "the program is ill-formed per N4950 [range.utility.conv.to]/2.3");

This doesn't help an average user to understand the problem.

It should be possible to give a more detailed message. What I have in mind:

if constexpr (_Ref_converts<_Rng, _Container>) {
    static_assert(false, "cannot find a way to create the result: "
                         "neither constructor call nor insertion at the end works");
} else {
    static_assert(
        false, "elements of the destination container must be "
               "implicitly convertible from elements of the source range");
}