validate.cpp: static_assert is_sorted() for the lookup tables

[StephanTLavavej]

When VS 2019 16.6 Preview 1 is available, the shipping toolset will support #6 constexpr is_sorted(). At that time, this will become a "good first issue".

1. We'll need to build the tools with /std:c++latest. This is accomplished by telling CMake to use C++20 instead of C++17:

STL/tools/CMakeLists.txt

Line 7 in a46d897

set(CMAKE_CXX_STANDARD 17)

2. Then we can remove this comment ("TRANSITION" means "TODO" for us), and change each assert to static_assert:

STL/tools/validate/validate.cpp

Lines 174 to 177 in a46d897

	// TRANSITION, P0202R3, use constexpr is_sorted()
	assert(is_sorted(skipped_directories.begin(), skipped_directories.end()));
	assert(is_sorted(skipped_extensions.begin(), skipped_extensions.end()));
	assert(is_sorted(tabby_filenames.begin(), tabby_filenames.end()));

(Related to #224.)

[BillyONeal]

There's no reason the tools can't use the live STL headers.

[StephanTLavavej]

Mixing live headers with installed libs would be doom. Doom!

[BillyONeal]

I mean, it wouldn't be a supported configuration but the only people it might break would be ourselves

[StephanTLavavej]

I'm wary of introducing little bits of weirdness - such things accumulate and make systems difficult to deal with. Waiting is simpler.

[BillyONeal]

Sure! Wasn't saying "do this thing", was only saying "consider this thing" :)

[bhardwajs]

@StephanTLavavej - I can work on this issue. Should I assign this to myself?

[StephanTLavavej]

Thanks! You can just submit a pull request - we don't usually assign issues to devs outside the vclibs team. A comment is sufficient to indicate to other readers that you're working on an issue.

[bhardwajs]

Great. Will submit a pull request.