dexonsmith added a comment.

I don't have access to rdar these days to look into the current state or to 
refresh my memory.

My memory is that the high-level goal was to move the libc++ headers from the 
toolchain to the SDK. For the transition, this was staged in such a way that 
they were in both locations for some period of time (internally, the transition 
may be ongoing, for all I know).

Apple clang needed to prefer the SDK, since that was the "new" place. I don't 
remember the reasoning for inverting this behaviour for LLVM clang.

- Maybe, because this way an LLVM toolchain could still override the C++ 
headers, like it could previously?
- Maybe, there was a plan to eventually switch the default in LLVM clang, but 
that got dropped/forgotten?
- Maybe, there was a plan to eventually switch the default in Apple clang, but 
that was dropped/forgotten, or the internal transition is ongoing? (This rings 
a bell. "Change LLVM clang to the desired end state, and we'll flip the 
direction temporarily in Apple clang for the duration of the transition...")

Anyway, I'm not sure the best way forward here. Perhaps clang should have a 
flag to decide which is preferred. Or perhaps LLVM clang should just change to 
match Apple clang for now.

@ldionne, thoughts?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
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https://reviews.llvm.org/D89001

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