Genex: Enable COMPILE_LANGUAGE for COMPILE_DEFINITIONS with VS and Xcode

The set of compile flags used for a target's C and C++ sources is based
on the linker language.  By default this is always the C++ flags if any
C++ sources appear in the target, and otherwise the C flags.  Therefore
we can define the `COMPILE_LANGUAGE` generator expression in
`COMPILE_DEFINITIONS` to match the selected language.

This is not exactly the same as for other generators, but is the best VS
and Xcode can do.  It is also sufficient for many use cases since the
set of definitions for C and C++ is frequently similar but may be
distinct from those for other languages like CUDA.

Issue: #17435

1 files changed, 6 insertions, 6 deletions

diff --git a/Source/cmGeneratorExpressionNode.cxx b/Source/cmGeneratorExpressionNode.cxx
index 3d311d6..597112a 100644
--- a/Source/cmGeneratorExpressionNode.cxx
+++ b/Source/cmGeneratorExpressionNode.cxx
@@ -828,20 +828,20 @@ static const struct CompileLanguageNode : public cmGeneratorExpressionNode
     }
     std::string genName = gg->GetName();
     if (genName.find("Visual Studio") != std::string::npos) {
-      if (dagChecker && (dagChecker->EvaluatingCompileDefinitions() ||
-                         dagChecker->EvaluatingIncludeDirectories())) {
+      if (dagChecker && dagChecker->EvaluatingIncludeDirectories()) {
         reportError(
           context, content->GetOriginalExpression(),
-          "$<COMPILE_LANGUAGE:...> may only be used for COMPILE_OPTIONS "
+          "$<COMPILE_LANGUAGE:...> may only be used for COMPILE_OPTIONS, "
+          "COMPILE_DEFINITIONS, "
           "and file(GENERATE) with the Visual Studio generator.");
         return std::string();
       }
     } else if (genName.find("Xcode") != std::string::npos) {
-      if (dagChecker && (dagChecker->EvaluatingCompileDefinitions() ||
-                         dagChecker->EvaluatingIncludeDirectories())) {
+      if (dagChecker && dagChecker->EvaluatingIncludeDirectories()) {
         reportError(
           context, content->GetOriginalExpression(),
-          "$<COMPILE_LANGUAGE:...> may only be used for COMPILE_OPTIONS "
+          "$<COMPILE_LANGUAGE:...> may only be used for COMPILE_OPTIONS, "
+          "COMPILE_DEFINITIONS, "
           "and file(GENERATE) with the Xcode generator.");
         return std::string();
       }