Explicitly specify language flag when source LANGUAGE property is set

Fixes: #14516, #20716

Tests/SetLang/CMakeLists.txt

@@ -8,3 +8,10 @@ add_executable(SetLang bar.c)
 set_source_files_properties(foo.c bar.c PROPERTIES LANGUAGE CXX)
 target_link_libraries(SetLang foo)
 set_target_properties(SetLang PROPERTIES LINKER_LANGUAGE CXX)
+
+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)
+  target_link_libraries(SetLang zoom)
+  target_compile_definitions(SetLang PRIVATE WITH_ZOOM)
+endif()

Tests/SetLang/bar.c

@@ -1,10 +1,22 @@
 #include <stdio.h>
 
 int foo();
+
+#ifdef WITH_ZOOM
+int zoom();
+#endif
+
 class A
 {
 public:
-  A() { this->i = foo(); }
+  A()
+  {
+    this->i = foo();
+#ifdef WITH_ZOOM
+    i += zoom();
+    i -= zoom();
+#endif
+  }
   int i;
 };
 

Tests/SetLang/zoom.zzz

@@ -0,0 +1,7 @@
+int zoom()
+{
+  int r = 10;
+  r++;
+  int ret = r + 10;
+  return ret;
+}