WG21-P2968R2 Make std::ignore A First-Class Object

No feature-test macro is mentioned.

We're already conforming to the improved specification (except for the location of std::ignore).

So the only things need to be done are

• moving _Ignore and ignore to <utility>,
• removing the /* strengthened */ comment from the operator=, and
• adding the // P2968R2 Make std::ignore A First-Class Object comment to <yvals_core.h> immediately after this line
  

  STL/stl/inc/yvals_core.h

  Line 81 in 165fc94

  // P2937R0 Freestanding Library: Remove strtok

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.

Please feel free to submit a pull request if there isn't one already linked here - no need to ask for permission! 😸

You can (and should) link your pull request to this issue using GitHub's close/fix/resolve syntax. (in the PR description not the commit message)