Fix Internal Compiler Error compiling Clang with the latest version of MSVC

[hughbe]

Summary

MSVC implodes compiling Clang, owing to the definition of llvm::None:

namespace llvm {
/// \brief A simple null object to allow implicit construction of Optional<T>
/// and similar types without having to spell out the specialization's name.
enum class NoneType { None };
const NoneType None = None;
}

It throws the following strange, obfuscated error:

Error: Unhandled exception at 0x00007FFFF9669633 (ntdll.dll) in cl.exe: 0xC0000005: Access violation writing location 0x00000036C7A00E90.

[1/665] Building CXX object tools\clang\lib\APINotes\CMakeFiles\clangAPINotes.dir\APINotesYAMLCompiler.cpp.obj
FAILED: tools/clang/lib/APINotes/CMakeFiles/clangAPINotes.dir/APINotesYAMLCompiler.cpp.obj
C:\PROGRA~2\MICROS~1.0\VC\bin\amd64\cl.exe   /nologo /TP -DCLANG_ENABLE_ARCMT -DCLANG_ENABLE_OBJC_REWRITER -DCLANG_ENABLE_STATIC_ANALYZER -DGTEST_HAS_RTTI=0 -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_GNU_SOURCE -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Itools\clang\lib\APINotes -IC:\Users\hbellamy\Documents\GitHub\llvm\tools\clang\lib\APINotes -IC:\Users\hbellamy\Documents\GitHub\llvm\tools\clang\include -Itools\clang\include -Iinclude -IC:\Users\hbellamy\Documents\GitHub\llvm\include /W4 -wd4141 -wd4146 -wd4180 -wd4244 -wd4258 -wd4267 -wd4291 -wd4345 -wd4351 -wd4355 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4800 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4324 -w14062 -we4238 /Zc:inline /Zc:strictStrings /Oi /Zc:rvalueCast -O2   -UNDEBUG  /EHs-c- /GR- /showIncludes /Fotools\clang\lib\APINotes\CMakeFiles\clangAPINotes.dir\APINotesYAMLCompiler.cpp.obj /Fdtools\clang\lib\APINotes\CMakeFiles\clangAPINotes.dir\ /FS -c C:\Users\hbellamy\Documents\GitHub\llvm\tools\clang\lib\APINotes\APINotesYAMLCompiler.cpp
Internal Compiler Error in C:\PROGRA~2\MICROS~1.0\VC\bin\amd64\cl.exe.  You will be prompted to send an error report to Microsoft later.

The fix is to give MSVC some more information. However, it still complains if we use llvm::None, so we have to use llvm::NoneType::None. I assume the problem is that MSVC gets horribly confused at this line: const NoneType None = None;

[malcolm.parsons]

This file doesn't exist in clang.
Did you mean to report this to swift-clang?

[hughbe]

Thanks Malcolm. You're right, my bad! Sorry for wasting your time

lib/APINotes/APINotesYAMLCompiler.cpp

     AvailabilityItem() : Mode(APIAvailability::Available), Msg("") {}
   };
 
-  static llvm::Optional<NullabilityKind> AbsentNullability = llvm::None;
+  // There is a bug in MSVC that causes an internal compiler error in compilation.
+  // See: https://connect.microsoft.com/VisualStudio/feedback/details/3111599/
+  // This is a workaround that issue until the problem gets fixed.
+#if defined(_MSC_VER)
+  static llvm::Optional<NullabilityKind> AbsentNullability = llvm::NoneType::None;
+#else
+  static llvm::Optional<NullabilityKind> AbsentNullability = None;
+#endif
+
   static llvm::Optional<NullabilityKind> DefaultNullability =
     NullabilityKind::NonNull;
   typedef std::vector<clang::NullabilityKind> NullabilitySeq;