AUTOMOC: Skip PCH when all sources files skip the PCH file too

Fixes: #23464

Source/cmQtAutoGenInitializer.cxx

@@ -1792,13 +1792,16 @@ void cmQtAutoGenInitializer::AddGeneratedSource(ConfigString const& filename,
   // XXX(xcode-per-cfg-src): Drop the Xcode-specific part of the condition
   // when the Xcode generator supports per-config sources.
   if (!this->MultiConfig || this->GlobalGen->IsXcode()) {
-    this->AddGeneratedSource(filename.Default, genVars, prepend);
+    cmSourceFile* sf =
+      this->AddGeneratedSource(filename.Default, genVars, prepend);
+    handleSkipPch(sf);
     return;
   }
   for (auto const& cfg : this->ConfigsList) {
     std::string const& filenameCfg = filename.Config.at(cfg);
     // Register source at makefile
-    this->RegisterGeneratedSource(filenameCfg);
+    cmSourceFile* sf = this->RegisterGeneratedSource(filenameCfg);
+    handleSkipPch(sf);
     // Add source file to target for this configuration.
     this->GenTarget->AddSource(
       cmStrCat("$<$<CONFIG:"_s, cfg, ">:"_s, filenameCfg, ">"_s), prepend);
@@ -2158,3 +2161,18 @@ bool cmQtAutoGenInitializer::GetQtExecutable(GenVarsT& genVars,
 
   return true;
 }
+
+void cmQtAutoGenInitializer::handleSkipPch(cmSourceFile* sf)
+{
+  bool skipPch = true;
+  for (auto const& pair : this->AutogenTarget.Sources) {
+    if (!pair.first->GetIsGenerated() &&
+        !pair.first->GetProperty("SKIP_PRECOMPILE_HEADERS")) {
+      skipPch = false;
+    }
+  }
+
+  if (skipPch) {
+    sf->SetProperty("SKIP_PRECOMPILE_HEADERS", "ON");
+  }
+}

Source/cmQtAutoGenInitializer.h

@@ -154,6 +154,8 @@ private:
   bool GetQtExecutable(GenVarsT& genVars, const std::string& executable,
                        bool ignoreMissingTarget) const;
 
+  void handleSkipPch(cmSourceFile* sf);
+
   cmQtAutoGenGlobalInitializer* GlobalInitializer = nullptr;
   cmGeneratorTarget* GenTarget = nullptr;
   cmGlobalGenerator* GlobalGen = nullptr;

Tests/RunCMake/Ninja/QtAutoMocSkipPch.cmake

@@ -0,0 +1,17 @@
+enable_language(CXX)
+
+set(QtX Qt${with_qt_version})
+
+find_package(${QtX} REQUIRED COMPONENTS Core)
+
+set(CMAKE_AUTOMOC ON)
+
+add_library(simple_lib SHARED simple_lib.cpp)
+add_executable(app_with_qt app.cpp app_qt.cpp)
+
+target_link_libraries(app_with_qt PRIVATE simple_lib ${QtX}::Core)
+
+set_source_files_properties(app.cpp app_qt.cpp
+    PROPERTIES SKIP_PRECOMPILE_HEADERS ON)
+
+target_precompile_headers(app_with_qt PRIVATE [["QObject"]])

Tests/RunCMake/Ninja/RunCMakeTest.cmake

@@ -357,6 +357,22 @@ function(run_QtAutoMocDeps)
     run_ninja("${RunCMake_TEST_BINARY_DIR}")
   endif()
 endfunction()
+
+function(run_QtAutoMocSkipPch)
+  set(QtX Qt${CMake_TEST_Qt_version})
+  if(CMake_TEST_${QtX}Core_Version VERSION_GREATER_EQUAL 5.15.0)
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/QtAutoMocSkipPch-build)
+    run_cmake_with_options(QtAutoMocSkipPch
+      "-Dwith_qt_version=${CMake_TEST_Qt_version}"
+      "-D${QtX}_DIR=${${QtX}_DIR}"
+      "-D${QtX}Core_DIR=${${QtX}Core_DIR}"
+      "-DCMAKE_PREFIX_PATH:STRING=${CMAKE_PREFIX_PATH}"
+    )
+    # Build the project.
+    run_ninja("${RunCMake_TEST_BINARY_DIR}")
+  endif()
+endfunction()
 if(CMake_TEST_Qt_version)
   run_QtAutoMocDeps()
+  run_QtAutoMocSkipPch()
 endif()