win: Enable /Zc:twoPhase by default if targeting MSVC 2017 update 3 or newer

MSVC 2017 update 3 (_MSC_VER 1911) enables /Zc:twoPhase by default, and
so should clang-cl:
https://docs.microsoft.com/en-us/cpp/build/reference/zc-twophase

clang-cl takes the MSVC version it emulates from the -fmsc-version flag,
or if that's not passed it tries to check what the installed version of
MSVC is and uses that, and failing that it uses a default version that's
currently 1911. So this changes the default if no -fmsc-version flag is
passed and no installed MSVC is detected. (It also changes the default
if -fmsc-version is passed or MSVC is detected, and either indicates
_MSC_VER >= 1911.)

As mentioned in the MSDN article, the Windows SDK header files in
version 10.0.15063.0 (Creators Update or Redstone 2) and earlier
versions do not work correctly with /Zc:twoPhase. If you need to use
these old SDKs with a new clang-cl, explicitly pass /Zc:twoPhase- to get
the old behavior.

Fixes PR43032.

Differential Revision: https://reviews.llvm.org/D66394

llvm-svn: 369402

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f3291137bca..0585438d751 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -90,6 +90,9 @@ Attribute Changes in Clang
 Windows Support
 ---------------
 
+- clang-cl now defaults to ``/Zc:twoPhase`` if targeting MSVC2017 update 3 or
+  later (``_MSC_VER`` >= 1911). This matches MSVC's behavior. Explicitly pass
+  ``/Zc:twoPhase-`` to restore the old behavior.
 - ...
 
 C Language Changes in Clang