P0439R0 enum class memory_order

[AFFogarty]

Description

This PR implements #17. memory_order becomes an enum class in cxx20.

Checklist:

• I understand README.md.
• If this is a feature addition, that feature has been voted into the C++
  Working Draft.
• Identifiers in any product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 .
• The STL builds and test harnesses have passed (must be manually verified
  by an STL maintainer before CI is online, leave this unchecked for initial
  submission). (Internal PR 204684)
• This change introduces no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate or
  trivially copyable, etc.). If unsure, leave this box unchecked and ask a
  maintainer for help.

[stevenhoving]

Just my 2 cents: it might be a good idea to squash your commits into a single commit.

[sylveon]

They can be squashed at merge time

[StephanTLavavej]

This feature needs to be mentioned in yvals_core.h in sorted order as being directly controlled by _HAS_CXX20:

STL/stl/inc/yvals_core.h

Line 21 in da76ab2

// _HAS_CXX20 directly controls:

Note that we use the GitHub issue title (which you used in your PR title, thanks!) for these comments, not the original paper title if it differs.

I think that we can manually test and merge this feature, since the test impact should be very small to nonexistent. Thank you!

[BillyONeal]

I think we want testing in place before merging this. I know the change is minor, and I think we could run the testing on your behalf, but I think we at least need a place for the tests to live first.

[AFFogarty]

CC: @imback82

[CaseyCarter]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

@CaseyCarter #114 needs to merge before that build will succeed.

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

/azp run

[azure-pipelines]

Pull request contains merge conflicts.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

Thanks for your contribution!

[CaseyCarter]

This is fine modulo test coverage.

[StephanTLavavej]

Finishing an internal build/test before I merge this.

[AFFogarty]

Thanks folks!