P1502 Standard library header units for C++20

[wg21bot]

P1502R0 Standard library header units for C++20 (Richard Smith)

[tituswinters]

This provides wording for what was asked for by LEWG in http://wiki.edg.com/bin/view/Wg21kona2019/P1453 - I believe there's no further design discussion for library, only wording review. Tweaking the labels accordingly.

[mclow]

LWG discussed this on Tuesday in Cologne. No consensus to move this as is, but strong consensus to move if the requirement to convert #include into import is removed (by P01881) .

[jensmaurer]

EWG in Cologne: "Consider it passed"

[wg21bot]

P1502R1 Standard library header units for C++20 (Richard Smith)