cmGlobalGenerator: factor out C++ module support checking

This will simplify adding support to other generators.

Source/cmGlobalGenerator.cxx

@@ -1439,6 +1439,19 @@ bool cmGlobalGenerator::CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const
   return false;
 }
 
+void cmGlobalGenerator::CxxModuleSupportCheck() const
+{
+  bool const diagnose = !this->DiagnosedCxxModuleSupport &&
+    !this->CMakeInstance->GetIsInTryCompile();
+  if (diagnose) {
+    this->DiagnosedCxxModuleSupport = true;
+    this->GetCMakeInstance()->IssueMessage(
+      MessageType::AUTHOR_WARNING,
+      "C++20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP "
+      "is experimental.  It is meant only for compiler developers to try.");
+  }
+}
+
 void cmGlobalGenerator::ComputeBuildFileGenerators()
 {
   for (unsigned int i = 0; i < this->LocalGenerators.size(); ++i) {
@@ -1597,6 +1610,8 @@ void cmGlobalGenerator::Generate()
   // it builds by default.
   this->InitializeProgressMarks();
 
+  this->DiagnosedCxxModuleSupport = false;
+
   this->ProcessEvaluationFiles();
 
   this->CMakeInstance->UpdateProgress("Generating", 0.1f);

Source/cmGlobalGenerator.h

@@ -157,6 +157,8 @@ public:
 
   virtual bool InspectConfigTypeVariables() { return true; }
 
+  virtual bool CheckCxxModuleSupport() { return false; }
+
   bool Compute();
   virtual void AddExtraIDETargets() {}
 
@@ -621,6 +623,8 @@ protected:
 
   virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const;
 
+  void CxxModuleSupportCheck() const;
+
   /// @brief Qt AUTOMOC/UIC/RCC target generation
   /// @return true on success
   bool QtAutoGen();
@@ -728,6 +732,8 @@ private:
   std::map<std::string, int> LanguageToLinkerPreference;
   std::map<std::string, std::string> LanguageToOriginalSharedLibFlags;
 
+  mutable bool DiagnosedCxxModuleSupport = false;
+
   // Deferral id generation.
   size_t NextDeferId = 0;
 

Source/cmGlobalNinjaGenerator.cxx

@@ -591,7 +591,7 @@ void cmGlobalNinjaGenerator::Generate()
   this->TargetAll = this->NinjaOutputPath("all");
   this->CMakeCacheFile = this->NinjaOutputPath("CMakeCache.txt");
   this->DisableCleandead = false;
-  this->DiagnosedCxxModuleSupport = false;
+  this->DiagnosedCxxModuleNinjaSupport = false;
 
   this->PolicyCMP0058 =
     this->LocalGenerators[0]->GetMakefile()->GetPolicyStatus(
@@ -850,18 +850,12 @@ bool cmGlobalNinjaGenerator::CheckLanguages(
 
 bool cmGlobalNinjaGenerator::CheckCxxModuleSupport()
 {
-  bool const diagnose = !this->DiagnosedCxxModuleSupport &&
-    !this->CMakeInstance->GetIsInTryCompile();
-  if (diagnose) {
-    this->DiagnosedCxxModuleSupport = true;
-    this->GetCMakeInstance()->IssueMessage(
-      MessageType::AUTHOR_WARNING,
-      "C++20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP "
-      "is experimental.  It is meant only for compiler developers to try.");
-  }
+  this->CxxModuleSupportCheck();
   if (this->NinjaSupportsDyndeps) {
     return true;
   }
+  bool const diagnose = !this->DiagnosedCxxModuleNinjaSupport &&
+    !this->CMakeInstance->GetIsInTryCompile();
   if (diagnose) {
     std::ostringstream e;
     /* clang-format off */

Source/cmGlobalNinjaGenerator.h

@@ -469,7 +469,7 @@ public:
 
   bool IsSingleConfigUtility(cmGeneratorTarget const* target) const;
 
-  bool CheckCxxModuleSupport();
+  bool CheckCxxModuleSupport() override;
 
 protected:
   void Generate() override;
@@ -592,7 +592,7 @@ private:
 
   codecvt::Encoding NinjaExpectedEncoding = codecvt::None;
 
-  bool DiagnosedCxxModuleSupport = false;
+  bool DiagnosedCxxModuleNinjaSupport = false;
 
   void InitOutputPathPrefix();