[PATCH] D139938: [clang] Don't spuriously pass -stdlib=libc++ to CC1 on Darwin

[Louis Dionne via Phabricator via cfe-commits]

ldionne created this revision.
ldionne added a reviewer: arphaman.
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Previously, we would be passing down -stdlib=libc++ from the Driver
to CC1 whenever the default standard library on the platform was libc++,
even if -stdlib= had not been passed to the Driver. This meant that we
would pass -stdlib=libc++ in nonsensical circumstances, such as when
compiling C code.

This logic had been added in b534ce46bd40 to make sure that header
search paths were set up properly. However, since libc++ is now the
default Standard Library on Darwin, passing this explicitly is not
required anymore. Indeed, if no -stdlib= is specified, CC1 will end
up using libc++ if it queries which standard library to use, without
having to be told.

Not passing -stdlib= at all to CC1 on Darwin should become possible
once CC1 stops relying on it to set up framework search paths.

Furthermore, this commit also removes a diagnostic checking whether the
deployment target is too old to support libc++. Nowadays, all supported
deployment targets use libc++ and compiling with libstdc++ is not
supported anymore. The Driver was the wrong place to issue this
diagnostic since it doesn't know whether libc++ will actually be linked
against (e.g. C vs C++), which would lead to spurious diagnostics.
Given that these targets are not supported anymore, we simply drop
the diagnostic instead of trying to refactor it into CC1.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139938

Files:
  clang/include/clang/Basic/DiagnosticDriverKinds.td
  clang/lib/Driver/ToolChains/Darwin.cpp
  clang/test/Driver/darwin-header-search-libcxx.cpp
  clang/test/Driver/darwin-stdlib-dont-pass-in-c.c
  clang/test/Driver/darwin-stdlib.cpp

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