Compile with explicit language flag when source LANGUAGE property is set

This change was originally made by commit 74b1c9fc8e (Explicitly specify
language flag when source LANGUAGE property is set, 2020-06-01,
v3.19.0-rc1~722^2), but it was reverted by commit 30aa715fac (Revert
"specify language flag when source LANGUAGE property is set",
2020-11-19) to restore compatibility with pre-3.19 behavior.

Implement the change again, but add policy CMP0119 to make this change
while preserving compatibility with existing projects.

Note that the `Compiler/{Clang,Intel,MSVC}-CXX` modules do not need to
specify `-TP` for their MSVC-like variants because we already use the
flag in `CMAKE_CXX_COMPILE_OBJECT`.  Similarly for `Compiler/XL-CXX`
and `Platform/Windows-Embarcadero`.

Note also that this does not seem possible to implement for XL C.
Even with `-qsourcetype=c`, `xlc` complains about an unknown suffix:
`1501-218 (W) file /.../AltExtC.zzz contains an incorrect file suffix`.
It returns non-zero even with `-qsuppress=1501-218`.

Co-Author: Robert Maynard <robert.maynard@kitware.com>
Fixes: #14516, #20716

1 files changed, 24 insertions, 0 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 4a79a3d..12f8e44 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -3155,6 +3155,30 @@ void cmGeneratorTarget::GetAppleArchs(const std::string& config,
   }
 }
 
+void cmGeneratorTarget::AddExplicitLanguageFlags(std::string& flags,
+                                                 cmSourceFile const& sf) const
+{
+  cmProp lang = sf.GetProperty("LANGUAGE");
+  if (!lang) {
+    return;
+  }
+
+  switch (this->GetPolicyStatusCMP0119()) {
+    case cmPolicies::WARN:
+    case cmPolicies::OLD:
+      // The OLD behavior is to not add explicit language flags.
+      return;
+    case cmPolicies::REQUIRED_ALWAYS:
+    case cmPolicies::REQUIRED_IF_USED:
+    case cmPolicies::NEW:
+      // The NEW behavior is to add explicit language flags.
+      break;
+  }
+
+  this->LocalGenerator->AppendFeatureOptions(flags, *lang,
+                                             "EXPLICIT_LANGUAGE");
+}
+
 void cmGeneratorTarget::AddCUDAArchitectureFlags(std::string& flags) const
 {
   const std::string& property = this->GetSafeProperty("CUDA_ARCHITECTURES");