find_package: Fixed CMAKE_FIND_PACKAGE_PREFER_CONFIG Module fallback

Fixes: #19361

Source/cmFindPackageCommand.cxx

@@ -502,8 +502,13 @@ bool cmFindPackageCommand::InitialPass(std::vector<std::string> const& args,
   if (this->Makefile->IsOn("CMAKE_FIND_PACKAGE_PREFER_CONFIG")) {
     if (this->UseConfigFiles && this->FindPackageUsingConfigMode()) {
       loadedPackage = true;
-    } else if (this->FindPackageUsingModuleMode()) {
-      loadedPackage = true;
+    } else {
+      if (this->FindPackageUsingModuleMode()) {
+        loadedPackage = true;
+      } else {
+        // The package was not loaded. Report errors.
+        HandlePackageMode(HandlePackageModeType::Module);
+      }
     }
   } else {
     if (this->UseFindModules && this->FindPackageUsingModuleMode()) {
@@ -603,7 +608,7 @@ bool cmFindPackageCommand::FindPackageUsingConfigMode()
   this->IgnoredPaths.insert(ignored.begin(), ignored.end());
 
   // Find and load the package.
-  return this->HandlePackageMode();
+  return this->HandlePackageMode(HandlePackageModeType::Config);
 }
 
 void cmFindPackageCommand::SetModuleVariables(const std::string& components)
@@ -722,7 +727,8 @@ bool cmFindPackageCommand::FindModule(bool& found)
   return true;
 }
 
-bool cmFindPackageCommand::HandlePackageMode()
+bool cmFindPackageCommand::HandlePackageMode(
+  HandlePackageModeType handlePackageModeType)
 {
   this->ConsideredConfigs.clear();
 
@@ -817,6 +823,12 @@ bool cmFindPackageCommand::HandlePackageMode()
     }
   }
 
+  if (this->Makefile->IsOn("CMAKE_FIND_PACKAGE_PREFER_CONFIG") && !found &&
+      handlePackageModeType == HandlePackageModeType::Config) {
+    // Config mode failed. Allow Module case.
+    result = false;
+  }
+
   // package not found
   if (result && !found) {
     // warn if package required or neither quiet nor in config mode

Source/cmFindPackageCommand.h

@@ -103,7 +103,14 @@ private:
   bool FindModule(bool& found);
   void AddFindDefinition(const std::string& var, const char* val);
   void RestoreFindDefinitions();
-  bool HandlePackageMode();
+
+  enum /*class*/ HandlePackageModeType
+  {
+    Module,
+    Config
+  };
+  bool HandlePackageMode(HandlePackageModeType type);
+
   bool FindConfig();
   bool FindPrefixedConfig();
   bool FindFrameworkConfig();

Tests/FindPackageTest/CMakeLists.txt

@@ -579,3 +579,16 @@ endif()
 
 set(CMAKE_FIND_PACKAGE_PREFER_CONFIG OFF)
 set(CMAKE_PREFIX_PATH)
+
+############################################################################
+##Test find_package CMAKE_FIND_PACKAGE_PREFER_CONFIG with module fallback
+
+set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/PreferConfigOnlyModule)
+
+set(CMAKE_FIND_PACKAGE_PREFER_CONFIG ON)
+
+find_package(ACME REQUIRED)
+
+if(NOT ACME_FOUND)
+    message(SEND_ERROR "Did not find ACME package")
+endif()

Tests/FindPackageTest/PreferConfigOnlyModule/FindACME.cmake

+set(ACME_FOUND TRUE)