Adopt LWG-3390

[Svido]

Fixes #547

Description

Adopting LWG-3390.

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[BillyONeal]

Can you add tests for this?

[Svido]

Can you add tests for this?
Sure. Where is the best place to add the tests?

[BillyONeal]

Can you add tests for this?
Sure. Where is the best place to add the tests?

Normally:

1. grep somewhere in tests/std for a test already testing close to what you're testing and insert into there where it makes sense
2. if there are no good candidates, add a new test by:
   • creating a new directory tests/std/tests/(name your test here)
   • put an env.lst in there indicating the configurations you support, in this case probably an identical copy of https://github.com/microsoft/STL/blob/master/tests/std/tests/P0595R2_is_constant_evaluated/env.lst
   • put a test.cpp in that directory that assert()s or static_assert()s what you want
   • add your new directory to tests/std/test.lst

[StephanTLavavej]

Perfect, I'll merge this to the Microsoft-internal repo. I don't think that we need specific test changes for this - testing should be added for WG21-P1207 as part of completing #39.

[StephanTLavavej]

Thanks again! Now we're closer to zero LWG issues remaining :-)