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

Tests/CMakeCommands/target_compile_definitions/CMakeLists.txt

@@ -26,18 +26,18 @@ target_compile_definitions(consumer
   PRIVATE
 )
 
-if (CMAKE_GENERATOR MATCHES "Makefiles" OR CMAKE_GENERATOR MATCHES "Ninja")
-  target_sources(consumer PRIVATE
-    "${CMAKE_CURRENT_SOURCE_DIR}/consumer.c"
-  )
+target_sources(consumer PRIVATE
+  "${CMAKE_CURRENT_SOURCE_DIR}/consumer.c"
+)
+target_compile_definitions(consumer
+  PRIVATE
+    CONSUMER_LANG_$<COMPILE_LANGUAGE>
+    LANG_IS_CXX=$<COMPILE_LANGUAGE:CXX>
+    LANG_IS_C=$<COMPILE_LANGUAGE:C>
+)
+if(CMAKE_GENERATOR MATCHES "Visual Studio|Xcode")
   target_compile_definitions(consumer
-    PRIVATE
-      CONSUMER_LANG_$<COMPILE_LANGUAGE>
-      LANG_IS_CXX=$<COMPILE_LANGUAGE:CXX>
-      LANG_IS_C=$<COMPILE_LANGUAGE:C>
-  )
-  target_compile_definitions(consumer
-    PRIVATE -DTEST_LANG_DEFINES
+    PRIVATE TEST_LANG_DEFINES_FOR_VISUAL_STUDIO_OR_XCODE
   )
 endif()
 

Tests/CMakeCommands/target_compile_definitions/consumer.c

@@ -1,5 +1,23 @@
 
-#ifdef TEST_LANG_DEFINES
+// Visual Studio allows only one set of flags for C and C++.
+// In a target using C++ we pick the C++ flags even for C sources.
+#ifdef TEST_LANG_DEFINES_FOR_VISUAL_STUDIO_OR_XCODE
+#ifndef CONSUMER_LANG_CXX
+#error Expected CONSUMER_LANG_CXX
+#endif
+
+#ifdef CONSUMER_LANG_C
+#error Unexpected CONSUMER_LANG_C
+#endif
+
+#if !LANG_IS_CXX
+#error Expected LANG_IS_CXX
+#endif
+
+#if LANG_IS_C
+#error Unexpected LANG_IS_C
+#endif
+#else
 #ifdef CONSUMER_LANG_CXX
 #error Unexpected CONSUMER_LANG_CXX
 #endif

Tests/CMakeCommands/target_compile_definitions/consumer.cpp

@@ -15,7 +15,6 @@
 #error Expected DASH_D_DEFINE
 #endif
 
-#ifdef TEST_LANG_DEFINES
 #ifndef CONSUMER_LANG_CXX
 #error Expected CONSUMER_LANG_CXX
 #endif
@@ -31,7 +30,6 @@
 #if LANG_IS_C
 #error Unexpected LANG_IS_C
 #endif
-#endif
 
 int main()
 {