[PATCH] D88984: Prefer libc++ headers in the -isysroot over the toolchain

[Duncan P. N. Exon Smith via Phabricator via cfe-commits]

dexonsmith added reviewers: rsmith, teemperor.
dexonsmith added a comment.

Hmm, that's right, to support user-provided libc++ headers it's simpler if we prefer the toolchain / next-to-clang.

The reason we need to invert it is transitional. The status quo is that we have no headers in the SDKs (either internally or publicly). We want to move the headers from the toolchain to the SDK. For [reasons], in some internal contexts we'll have headers both places for a small number of years, where the SDK headers should be preferred if both exist.

Some ideas:

- Add a command-line option to decide whether to check the SDK or next-to-clang first. Maybe we can restrict it to a `-cc1` option.
- Make the default behaviour configurable, and potentially use a different default in the Apple CMake cache.

WDYT?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D88984/new/

https://reviews.llvm.org/D88984