Merge branch 'backport-vs-lang-flags' into vs-lang-flags

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2919,6 +2919,12 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
     clOptions.RemoveFlag("SpectreMitigation");
   }
 
+  // Remove any target-wide -TC or -TP flag added by the project.
+  // Such flags are unnecessary and break our model of language selection.
+  if (langForClCompile == "C" || langForClCompile == "CXX") {
+    clOptions.RemoveFlag("CompileAs");
+  }
+
   this->ClOptions[configName] = std::move(pOptions);
   return true;
 }

Tests/SetLang/CMakeLists.txt

@@ -9,6 +9,13 @@ set_source_files_properties(foo.c bar.c PROPERTIES LANGUAGE CXX)
 target_link_libraries(SetLang foo)
 set_target_properties(SetLang PROPERTIES LINKER_LANGUAGE CXX)
 
+# VS generators historically tolerated target-wide -TP flags added
+# by project code, so cover that case to preserve the behavior.
+if(CMAKE_GENERATOR MATCHES "^Visual Studio" AND "x${CMAKE_C_COMPILER_ID}" STREQUAL "xMSVC")
+  add_library(stay stay_c.c stay_cxx.cxx)
+  set_property(TARGET stay PROPERTY COMPILE_OPTIONS "-TP")
+endif()
+
 if((CMAKE_C_COMPILER_ID MATCHES "(GNU|Clang|MSVC|Borland|Embarcadero|Intel|TI|XL)"))
   add_library(zoom zoom.zzz)
   set_source_files_properties(zoom.zzz PROPERTIES LANGUAGE CXX)

Tests/SetLang/stay_c.c

@@ -0,0 +1,8 @@
+#ifdef __cplusplus
+#  error C source incorrectly compiled as C++
+#endif
+
+int stay_c(void)
+{
+  return 0;
+}

Tests/SetLang/stay_cxx.cxx

@@ -0,0 +1,8 @@
+#ifndef __cplusplus
+#  error C++ source incorrectly compiled as C
+#endif
+
+int stay_cxx()
+{
+  return 0;
+}