Use C++20 library feature tests

[purpleKarrot]

The following code breaks when it is compiled with a C++20 capable compiler without using a C++20 capable standard library:

#if nsel_CPP20_OR_GREATER

using std::remove_cvref;

#else

Please don't assume that C++20 library features are available depending on the language standard that was selected with -std=.
Instead, check for each feature individually, as in

#if defined(__cpp_lib_remove_cvref)

using std::remove_cvref;

#else

See the list of feature tests here: https://en.cppreference.com/w/cpp/feature_test

[striezel]

Looks like this issue has been fixed in commit 34524d4 / release 0.6.2, so this issue could be closed.

[martinmoene]

Thanks @striezel !