Use unqualified function calls for valarray

[miscco]

Description

The standard requires that the transcend functions that are applied unqualified to each element of valarray.

This addresses #285

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 and
  the C++ Working Draft as a reference (and any other cited standards).
  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.

[CaseyCarter]

[valarray,transcend]/1 is not the clearest paragraph of wording in the Standard. This paragraph came up while discussing Revision 2 of WG21-P1719 which changes the Requires to Mandates in LWG last week. We determined (1) this wording unclear, (2) we're not extremely motivated to cleanup valarray wording, and (3) we can't devise a sensible reading for this wording other than that it intends unqualified invocations which result in ADL.

TLDR: I agree with the intent of the P/R and agree it addresses the issue.

[CaseyCarter]

Looks good modulo test coverage; we should have a test to validate the library finds functions via ADL.

Other maintainers: Should we comment these callsites to make it clear these unqualified calls are intentional?

[CaseyCarter]

/azp run

EDIT: Woops - I forgot the checks are now automatic and I don't need to poke the bot.

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

Other maintainers: Should we comment these callsites to make it clear these unqualified calls are intentional?

This is sufficiently unusual that I think comments are warranted, yes.

[StephanTLavavej]

Looks good to me. I went ahead and added comments, following the precedent of

STL/stl/inc/system_error

Line 122 in f9b1dcc

*this = make_error_code(_Errcode); // using ADL

I don't think test coverage for this is necessary; we'd have to write "fancy numeric types". The comments should be sufficient to ensure that we don't add qualification during maintenance.

[StephanTLavavej]

I'm porting this to MSVC-internal git now, thanks.

[StephanTLavavej]

Thanks again for the bugfix! :-)