clang-cl: Enable /Zc:twoPhase by default if targeting MSVC 2017 update 3 or newer

[thakis]

Summary

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.

[rnk]

lgtm! I can't believe we didn't notice this for a year. Not long ago a Firefox developer filed a bug for a crash with -fdelayed-template-parsing, and if this works for them, we won't have to debug it. :)

[hans]

Nice! Please include a mention in docs/ReleaseNotes.rst :-)

cfe/trunk/docs/ReleaseNotes.rst

 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
 ---------------------------
 
 - ...
 
 C11 Feature Support

cfe/trunk/include/clang/Driver/CLCompatOptions.td

 def _SLASH_Zc_threadSafeInit_ : CLFlag<"Zc:threadSafeInit-">,
   HelpText<"Disable thread-safe initialization of static variables">,
   Alias<fno_threadsafe_statics>;
 def _SLASH_Zc_trigraphs : CLFlag<"Zc:trigraphs">,
   HelpText<"Enable trigraphs">, Alias<ftrigraphs>;
 def _SLASH_Zc_trigraphs_off : CLFlag<"Zc:trigraphs-">,
   HelpText<"Disable trigraphs (default)">, Alias<fno_trigraphs>;
 def _SLASH_Zc_twoPhase : CLFlag<"Zc:twoPhase">,
-  HelpText<"Enable two-phase name lookup in templates">,
+  HelpText<"Enable two-phase name lookup in templates (default)">,
   Alias<fno_delayed_template_parsing>;
 def _SLASH_Zc_twoPhase_ : CLFlag<"Zc:twoPhase-">,
-  HelpText<"Disable two-phase name lookup in templates (default)">,
+  HelpText<"Disable two-phase name lookup in templates">,
   Alias<fdelayed_template_parsing>;
 def _SLASH_Z7 : CLFlag<"Z7">,
   HelpText<"Enable CodeView debug information in object files">;
 def _SLASH_Zd : CLFlag<"Zd">,
   HelpText<"Emit debug line number tables only">;
 def _SLASH_Zi : CLFlag<"Zi">, Alias<_SLASH_Z7>,
   HelpText<"Like /Z7">;
 def _SLASH_Zp : CLJoined<"Zp">,

cfe/trunk/lib/Driver/ToolChains/Clang.cpp

 
   // -fthreadsafe-static is default, except for MSVC compatibility versions less
   // than 19.
   if (!Args.hasFlag(options::OPT_fthreadsafe_statics,
                     options::OPT_fno_threadsafe_statics,
                     !IsWindowsMSVC || IsMSVC2015Compatible))
     CmdArgs.push_back("-fno-threadsafe-statics");
 
-  // -fno-delayed-template-parsing is default, except when targeting MSVC.
-  // Many old Windows SDK versions require this to parse.
-  // FIXME: MSVC introduced /Zc:twoPhase- to disable this behavior in their
-  // compiler. We should be able to disable this by default at some point.
+  // -fno-delayed-template-parsing is default, except when targeting MSVC
+  // earlier than MSVC 2017 15.3 (_MSC_VER 1911).  Windows SDK versions
+  // 10.0.15063.0 (Creators Update or Redstone 2) and earlier require this to
+  // parse.
+  bool IsMSVCBefore2017Update3 = !MSVT.empty() && MSVT < VersionTuple(19, 11);
   if (Args.hasFlag(options::OPT_fdelayed_template_parsing,
-                   options::OPT_fno_delayed_template_parsing, IsWindowsMSVC))
+                   options::OPT_fno_delayed_template_parsing,
+                   IsMSVCBefore2017Update3))
     CmdArgs.push_back("-fdelayed-template-parsing");
 
   // -fgnu-keywords default varies depending on language; only pass if
   // specified.
   Args.AddLastArg(CmdArgs, options::OPT_fgnu_keywords,
                   options::OPT_fno_gnu_keywords);
 
   if (Args.hasFlag(options::OPT_fgnu89_inline, options::OPT_fno_gnu89_inline,

cfe/trunk/test/Driver/cl-options.c

 // DEFAULTSTRICT: "-relaxed-aliasing"
 // RUN: %clang_cl -c -fstrict-aliasing -### -- %s 2>&1 | FileCheck -check-prefix=STRICT %s
 // STRICT-NOT: "-relaxed-aliasing"
 // RUN: %clang_cl -c -fno-strict-aliasing -### -- %s 2>&1 | FileCheck -check-prefix=NOSTRICT %s
 // NOSTRICT: "-relaxed-aliasing"
 
 // We recognize -f[no-]delayed-template-parsing.
 // /Zc:twoPhase[-] has the opposite meaning.
-// RUN: %clang_cl -c -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDDEFAULT %s
-// DELAYEDDEFAULT: "-fdelayed-template-parsing"
-// RUN: %clang_cl -c -fdelayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
-// RUN: %clang_cl -c /Zc:twoPhase- -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
+// RUN: %clang_cl -c -fmsc-version=1910 -### -- %s 2>&1 | FileCheck -check-prefix=OLDDELAYEDDEFAULT %s
+// OLDDELAYEDDEFAULT: "-fdelayed-template-parsing"
+// RUN: %clang_cl -fmsc-version=1911 -c -### -- %s 2>&1 | FileCheck -check-prefix=NEWDELAYEDDEFAULT %s
+// NEWDELAYEDDEFAULT-NOT: "-fdelayed-template-parsing"
+// RUN: %clang_cl -c -fmsc-version=1910 -fdelayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
+// RUN: %clang_cl -c -fmsc-version=1911 -fdelayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
+// RUN: %clang_cl -c -fmsc-version=1910 /Zc:twoPhase- -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
+// RUN: %clang_cl -c -fmsc-version=1911 /Zc:twoPhase- -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDON %s
 // DELAYEDON: "-fdelayed-template-parsing"
-// RUN: %clang_cl -c -fno-delayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
-// RUN: %clang_cl -c /Zc:twoPhase -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
+// RUN: %clang_cl -c -fmsc-version=1910 -fno-delayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
+// RUN: %clang_cl -c -fmsc-version=1911 -fno-delayed-template-parsing -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
+// RUN: %clang_cl -c -fmsc-version=1910 /Zc:twoPhase -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
+// RUN: %clang_cl -c -fmsc-version=1911 /Zc:twoPhase -### -- %s 2>&1 | FileCheck -check-prefix=DELAYEDOFF %s
 // DELAYEDOFF-NOT: "-fdelayed-template-parsing"
 
 // RUN: %clang_cl -c -### /std:c++latest -- %s 2>&1 | FileCheck -check-prefix CHECK-LATEST-CHAR8_T %s
 // CHECK-LATEST-CHAR8_T-NOT: "-fchar8_t"
 // RUN: %clang_cl -c -### /Zc:char8_t -- %s 2>&1 | FileCheck -check-prefix CHECK-CHAR8_T %s
 // CHECK-CHAR8_T: "-fchar8_t"
 // RUN: %clang_cl -c -### /Zc:char8_t- -- %s 2>&1 | FileCheck -check-prefix CHECK-CHAR8_T_ %s
 // CHECK-CHAR8_T_: "-fno-char8_t"