VS: Restore compilation of '.C' sources as C++

Refactoring in commit 3b547e2e4b (VS: Simplify logic adding source file
C/C++ language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) failed to
account for MSVC's treatment of `.C` extensions as C.  Add this special
case to the logic to restore use of `-TP` for `.C` extensions.

Fixes: #20822

1 files changed, 6 insertions, 3 deletions

diff --git a/Source/cmVisualStudio10TargetGenerator.cxx b/Source/cmVisualStudio10TargetGenerator.cxx
index 97c4c856b0..a3ccd2baee 100644
--- a/Source/cmVisualStudio10TargetGenerator.cxx
+++ b/Source/cmVisualStudio10TargetGenerator.cxx
@@ -2321,11 +2321,14 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
   }
 
   // Force language if the file extension does not match.
+  // Note that MSVC treats the upper-case '.C' extension as C and not C++.
+  std::string const ext = sf.GetExtension();
+  std::string const extLang = ext == "C"
+    ? "C"
+    : this->GlobalGenerator->GetLanguageFromExtension(ext.c_str());
   std::string lang = this->LocalGenerator->GetSourceFileLanguage(sf);
   const char* compileAs = 0;
-  if (lang !=
-      this->GlobalGenerator->GetLanguageFromExtension(
-        sf.GetExtension().c_str())) {
+  if (lang != extLang) {
     if (lang == "CXX") {
       // force a C++ file type
       compileAs = "CompileAsCpp";