VS: Restore toleration of target-wide -TP flag with MSVC

Since commit 3b547e2e4b (VS: Simplify logic adding source file C/C++
language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) we only add a
per-source language selection flag when the source file extension does
not match the compiler's default.  This approach breaks when a project
adds a target-wide `-TP` flag.

Although such projects likely did not work with non-VS generators, we
did support them before in Visual Studio generators.  Add a special case
to tolerate such flags again.

Fixes: #21005

1 files changed, 6 insertions, 0 deletions

diff --git a/Source/cmVisualStudio10TargetGenerator.cxx b/Source/cmVisualStudio10TargetGenerator.cxx
index a3ccd2b..a8e424d 100644
--- a/Source/cmVisualStudio10TargetGenerator.cxx
+++ b/Source/cmVisualStudio10TargetGenerator.cxx
@@ -2884,6 +2884,12 @@ 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;
 }