Explicitly specify language flag when source LANGUAGE property is set

Fixes: #14516, #20716

Source/cmExtraSublimeTextGenerator.cxx

@@ -349,6 +349,13 @@ std::string cmExtraSublimeTextGenerator::ComputeFlagsForObject(
   if (language.empty()) {
     language = "C";
   }
+
+  // explicitly add the explicit language flag before any other flag
+  // this way backwards compatibility with user flags is maintained
+  if (source->GetProperty("LANGUAGE")) {
+    lg->AppendFeatureOptions(flags, language, "EXPLICIT_LANGUAGE");
+  }
+
   std::string const& config =
     lg->GetMakefile()->GetSafeDefinition("CMAKE_BUILD_TYPE");
 

Source/cmGlobalXCodeGenerator.cxx

@@ -819,6 +819,14 @@ cmXCodeObject* cmGlobalXCodeGenerator::CreateXCodeSourceFile(
     default:
       break;
   }
+
+  // explicitly add the explicit language flag before any other flag
+  // this way backwards compatibility with user flags is maintained
+  if (sf->GetProperty("LANGUAGE")) {
+    this->CurrentLocalGenerator->AppendFeatureOptions(flags, lang,
+                                                      "EXPLICIT_LANGUAGE");
+  }
+
   const std::string COMPILE_FLAGS("COMPILE_FLAGS");
   if (cmProp cflags = sf->GetProperty(COMPILE_FLAGS)) {
     lg->AppendFlags(flags, genexInterpreter.Evaluate(*cflags, COMPILE_FLAGS));

Source/cmMakefileTargetGenerator.cxx

@@ -532,6 +532,13 @@ void cmMakefileTargetGenerator::WriteObjectRuleFiles(
   // Build the set of compiler flags.
   std::string flags;
 
+  // explicitly add the explicit language flag before any other flag
+  // this way backwards compatibility with user flags is maintained
+  if (source.GetProperty("LANGUAGE")) {
+    this->LocalGenerator->AppendFeatureOptions(flags, lang,
+                                               "EXPLICIT_LANGUAGE");
+  }
+
   // Add language-specific flags.
   std::string langFlags = cmStrCat("$(", lang, "_FLAGS", filterArch, ")");
   this->LocalGenerator->AppendFlags(flags, langFlags);

Source/cmNinjaTargetGenerator.cxx

@@ -188,7 +188,16 @@ std::string cmNinjaTargetGenerator::ComputeFlagsForObject(
     }
   }
 
-  std::string flags = this->GetFlags(language, config, filterArch);
+  std::string flags;
+  // explicitly add the explicit language flag before any other flag
+  // this way backwards compatibility with user flags is maintained
+  if (source->GetProperty("LANGUAGE")) {
+    this->LocalGenerator->AppendFeatureOptions(flags, language,
+                                               "EXPLICIT_LANGUAGE");
+    flags += " ";
+  }
+
+  flags += this->GetFlags(language, config, filterArch);
 
   // Add Fortran format flags.
   if (language == "Fortran") {

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;
+}