Autogen: Skip generated files for compatibility with CMake 3.8

The change in commit v3.9.0-rc1~464^2~8 (Autogen: Add AUTOMOC/UIC
support for generated source files, 2017-03-02) changes behavior of
existing projects that may not expect `AUTOGEN` on generated files and
do not yet set `SKIP_AUTOGEN` on them.  Disable the behavior change for
now to fix the regression for CMake 3.9.  We can restore it later with a
policy.

In order to keep the implementation and tests working, add an
undocumented property we can use in the tests to enable the behavior
before the policy is introduced.

Fixes: #17031
Issue: #16186

Source/cmQtAutoGeneratorInitializer.cxx

@@ -303,6 +303,13 @@ static void AcquireScanFiles(cmGeneratorTarget const* target,
         !(fileType == cmSystemTools::HEADER_FILE_FORMAT)) {
       continue;
     }
+    if (PropertyEnabled(sf, "GENERATED") &&
+        !target->GetPropertyAsBool("__UNDOCUMENTED_AUTOGEN_GENERATED_FILES")) {
+      // FIXME: Add a policy whose NEW behavior allows generated files.
+      // The implementation already works.  We disable it here to avoid
+      // changing behavior for existing projects that do not expect it.
+      continue;
+    }
     const std::string absFile =
       cmsys::SystemTools::GetRealPath(sf->GetFullPath());
     // Skip flags

Tests/QtAutogen/mocDepends/CMakeLists.txt

@@ -28,6 +28,7 @@ add_executable(mocDepends1 test1.cpp
 )
 target_link_libraries(mocDepends1 ${QT_CORE_TARGET})
 set_target_properties(mocDepends1 PROPERTIES AUTOMOC TRUE)
+set_property(TARGET mocDepends1 PROPERTY __UNDOCUMENTED_AUTOGEN_GENERATED_FILES 1)
 
 # -- Test 2 using generated library
 # This tests the dependency of AUTOMOC of mocDepends2 to the
@@ -43,3 +44,4 @@ add_library(SimpleLib STATIC simpleLib.hpp simpleLib.cpp)
 add_executable(mocDepends2 test2.cpp )
 target_link_libraries(mocDepends2 SimpleLib ${QT_CORE_TARGET})
 set_target_properties(mocDepends2 PROPERTIES AUTOMOC TRUE)
+set_property(TARGET mocDepends2 PROPERTY __UNDOCUMENTED_AUTOGEN_GENERATED_FILES 1)

Tests/QtAutogen/mocRerun/CMakeLists.txt

@@ -27,6 +27,7 @@ add_executable(mocRerun
   ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
   res1.qrc
   )
+set_property(TARGET mocRerun PROPERTY __UNDOCUMENTED_AUTOGEN_GENERATED_FILES 1)
 target_include_directories(mocRerun PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
 target_link_libraries(mocRerun ${QT_CORE_TARGET})
 # Write target name to text file