cmSourceFile: Rename non-const GetLanguage

GetOrDetermineLanguage:
- Read the property if available
- Determine the Language using the file extension

Fix all usage of the non-const member in the repository.

Source/cmExtraCodeBlocksGenerator.cxx

@@ -365,7 +365,7 @@ void cmExtraCodeBlocksGenerator::CreateNewProjectFile(
 
             // check whether it is a C/C++/CUDA implementation file
             bool isCFile = false;
-            std::string lang = s->GetLanguage();
+            std::string lang = s->GetOrDetermineLanguage();
             if (lang == "C" || lang == "CXX" || lang == "CUDA") {
               std::string const& srcext = s->GetExtension();
               isCFile = cm->IsSourceExtension(srcext);

Source/cmExtraSublimeTextGenerator.cxx

@@ -342,7 +342,7 @@ std::string cmExtraSublimeTextGenerator::ComputeFlagsForObject(
   cmSourceFile* source, cmLocalGenerator* lg, cmGeneratorTarget* gtgt)
 {
   std::string flags;
-  std::string language = source->GetLanguage();
+  std::string language = source->GetOrDetermineLanguage();
   if (language.empty()) {
     language = "C";
   }
@@ -377,7 +377,7 @@ std::string cmExtraSublimeTextGenerator::ComputeDefines(
 {
   std::set<std::string> defines;
   cmMakefile* makefile = lg->GetMakefile();
-  const std::string& language = source->GetLanguage();
+  const std::string& language = source->GetOrDetermineLanguage();
   const std::string& config = makefile->GetSafeDefinition("CMAKE_BUILD_TYPE");
   cmGeneratorExpressionInterpreter genexInterpreter(lg, config, target,
                                                     language);
@@ -410,7 +410,7 @@ std::string cmExtraSublimeTextGenerator::ComputeIncludes(
 {
   std::vector<std::string> includes;
   cmMakefile* makefile = lg->GetMakefile();
-  const std::string& language = source->GetLanguage();
+  const std::string& language = source->GetOrDetermineLanguage();
   const std::string& config = makefile->GetSafeDefinition("CMAKE_BUILD_TYPE");
   cmGeneratorExpressionInterpreter genexInterpreter(lg, config, target,
                                                     language);

Source/cmFileAPICodemodel.cxx

@@ -837,7 +837,7 @@ CompileData Target::BuildCompileData(cmSourceFile* sf)
 {
   CompileData fd;
 
-  fd.Language = sf->GetLanguage();
+  fd.Language = sf->GetOrDetermineLanguage();
   if (fd.Language.empty()) {
     return fd;
   }

Source/cmGeneratorTarget.cxx

@@ -1593,7 +1593,7 @@ void cmGeneratorTarget::ComputeKindedSources(KindedSources& files,
       kind = SourceKindHeader;
     } else if (sf->GetPropertyAsBool("EXTERNAL_OBJECT")) {
       kind = SourceKindExternalObject;
-    } else if (!sf->GetLanguage().empty()) {
+    } else if (!sf->GetOrDetermineLanguage().empty()) {
       kind = SourceKindObjectSource;
     } else if (ext == "def") {
       kind = SourceKindModuleDefinition;
@@ -6055,7 +6055,7 @@ void cmGeneratorTarget::GetLanguages(std::set<std::string>& languages,
   std::vector<cmSourceFile*> sourceFiles;
   this->GetSourceFiles(sourceFiles, config);
   for (cmSourceFile* src : sourceFiles) {
-    const std::string& lang = src->GetLanguage();
+    const std::string& lang = src->GetOrDetermineLanguage();
     if (!lang.empty()) {
       languages.insert(lang);
     }

Source/cmGetSourceFilePropertyCommand.cxx

@@ -25,7 +25,7 @@ bool cmGetSourceFilePropertyCommand::InitialPass(
   }
   if (sf) {
     if (args[2] == "LANGUAGE") {
-      this->Makefile->AddDefinition(var, sf->GetLanguage());
+      this->Makefile->AddDefinition(var, sf->GetOrDetermineLanguage());
       return true;
     }
     const char* prop = nullptr;

Source/cmJsonObjects.cxx

@@ -264,7 +264,7 @@ static Json::Value DumpSourceFilesList(
   std::unordered_map<LanguageData, std::vector<std::string>> fileGroups;
   for (cmSourceFile* file : files) {
     LanguageData fileData;
-    fileData.Language = file->GetLanguage();
+    fileData.Language = file->GetOrDetermineLanguage();
     if (!fileData.Language.empty()) {
       const LanguageData& ld = languageDataMap.at(fileData.Language);
       cmLocalGenerator* lg = target->GetLocalGenerator();

Source/cmSourceFile.cxx

@@ -45,11 +45,13 @@ std::string cmSourceFile::GetObjectLibrary() const
   return this->ObjectLibrary;
 }
 
-std::string cmSourceFile::GetLanguage()
+std::string const& cmSourceFile::GetOrDetermineLanguage()
 {
   // If the language was set explicitly by the user then use it.
   if (const char* lang = this->GetProperty(propLANGUAGE)) {
-    return lang;
+    // Assign to member in order to return a reference.
+    this->Language = lang;
+    return this->Language;
   }
 
   // Perform computation needed to get the language if necessary.
@@ -72,8 +74,8 @@ std::string cmSourceFile::GetLanguage()
     }
   }
 
-  // Now try to determine the language.
-  return static_cast<cmSourceFile const*>(this)->GetLanguage();
+  // Use the language determined from the file extension.
+  return this->Language;
 }
 
 std::string cmSourceFile::GetLanguage() const
@@ -83,13 +85,8 @@ std::string cmSourceFile::GetLanguage() const
     return lang;
   }
 
-  // If the language was determined from the source file extension use it.
-  if (!this->Language.empty()) {
-    return this->Language;
-  }
-
-  // The language is not known.
-  return "";
+  // Use the language determined from the file extension.
+  return this->Language;
 }
 
 cmSourceFileLocation const& cmSourceFile::GetLocation() const

Source/cmSourceFile.h

@@ -88,7 +88,7 @@ public:
   /**
    * Get the language of the compiler to use for this source file.
    */
-  std::string GetLanguage();
+  std::string const& GetOrDetermineLanguage();
   std::string GetLanguage() const;
 
   /**