VS: Do not compile C sources as WinRT (#15100)

The MSVC /ZW flag is valid only for C++ sources.  Whenever we enable
CompileAsWinRT for the whole target, disable it for all C sources.
Update the documentation of VS_WINRT_COMPONENT to drop the statement
about undefined behavior for non-C++ sources, because it is now
defined as expected.

Help/prop_tgt/VS_WINRT_COMPONENT.rst

@@ -7,8 +7,5 @@ For ``SHARED`` and ``MODULE`` libraries, this also defines the
 ``_WINRT_DLL`` preprocessor macro.
 
 .. note::
-  Behavior is not defined for targets with source files that compile as
-  any language other than ``CXX``.
-
   Currently this is implemented only by Visual Studio generators.
   Support may be added to other generators in the future.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -159,6 +159,7 @@ cmVisualStudio10TargetGenerator(cmTarget* target,
   this->GUID = this->GlobalGenerator->GetGUID(this->Name.c_str());
   this->Platform = gg->GetPlatformName();
   this->MSTools = true;
+  this->TargetCompileAsWinRT = false;
   this->BuildFileStream = 0;
   this->IsMissingFiles = false;
   this->DefaultArtifactDir =
@@ -1414,6 +1415,7 @@ bool cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
       compileAs = "CompileAsC";
       }
     }
+  bool noWinRT = this->TargetCompileAsWinRT && lang == "C";
   bool hasFlags = false;
   // for the first time we need a new line if there is something
   // produced here.
@@ -1447,7 +1449,7 @@ bool cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
       }
     // if we have flags or defines for this config then
     // use them
-    if(!flags.empty() || !configDefines.empty() || compileAs)
+    if(!flags.empty() || !configDefines.empty() || compileAs || noWinRT)
       {
       (*this->BuildFileStream ) << firstString;
       firstString = ""; // only do firstString once
@@ -1460,6 +1462,10 @@ bool cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
         {
         clOptions.AddFlag("CompileAs", compileAs);
         }
+      if(noWinRT)
+        {
+        clOptions.AddFlag("CompileAsWinRT", "false");
+        }
       clOptions.Parse(flags.c_str());
       clOptions.AddDefines(configDefines.c_str());
       clOptions.SetConfiguration((*config).c_str());
@@ -1711,6 +1717,13 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
         clOptions.AddFlag("CompileAsWinRT", "false");
         }
       }
+    if(const char* winRT = clOptions.GetFlag("CompileAsWinRT"))
+      {
+      if(cmSystemTools::IsOn(winRT))
+        {
+        this->TargetCompileAsWinRT = true;
+        }
+      }
     }
 
   this->ClOptions[configName] = pOptions.release();

Source/cmVisualStudio10TargetGenerator.h

@@ -134,6 +134,7 @@ private:
   std::string GUID;
   std::string Name;
   bool MSTools;
+  bool TargetCompileAsWinRT;
   cmGlobalVisualStudio10Generator* GlobalGenerator;
   cmGeneratedFileStream* BuildFileStream;
   cmLocalVisualStudio7Generator* LocalGenerator;