C++20 removed std::is_literal_type

[StephanTLavavej]

std::is_literal_type was deprecated in C++17 and removed in C++20. Section [diff.cpp17.depr]/7 of the C++20 Working Draft explains:

The traits had unreliable or awkward interfaces. The is_­literal_­type trait provided no way to detect which subset of constructors and member functions of a type were declared constexpr.

This trait is being directly used here:

https://github.com/bolero-MURAKAMI/Sprout/blob/6b5addba9face0a6403e66e7db2aa94d87387f61/sprout/type_traits/is_literal_type.hpp#L21

In /std:c++17 mode, this triggers a deprecation warning in recent versions of MSVC. In /std:c++latest mode, now that https://github.com/microsoft/STL/pull/380 has been merged, this will trigger an error in VS 2019 16.6 Preview 2.

MSVC provides "escape hatch" macros that can be defined project-wide to suppress the deprecation warning and restore the removed type trait. (Specifically, compiling with /D_SILENCE_CXX17_IS_LITERAL_TYPE_DEPRECATION_WARNING and /D_HAS_DEPRECATED_IS_LITERAL_TYPE=1.) However, it's best to avoid using this deprecated/removed machinery entirely.

I see at least three uses of sprout::is_literal_type throughout your codebase, but I'm uncertain as to how it's being used. I looked at prev, where it appears to be dispatching to logarithmic-depth recursion, possibly as a workaround when C++14 extended constexpr is unavailable. If all uses of is_literal_type are for dealing with the limitations of C++11 classic constexpr, you may want to detect whether C++14 extended constexpr is available, and if so, avoid is_literal_type entirely (which should solve the deprecation/removal issue with the latest versions of MSVC).