Autogen: Restore mocs_compilation in OBJECT libraries

Since commit f65f20938c (Autogen: Avoid processing CSharp targets,
2020-11-12, v3.20.0-rc1~301^2) we collect all sources for a target
earlier than previously.  Clear the sources cache so that it will be
re-computed later after AUTOGEN processing.

Fixes: #22085

Source/cmQtAutoGenGlobalInitializer.cxx

@@ -97,6 +97,11 @@ cmQtAutoGenGlobalInitializer::cmQtAutoGenGlobalInitializer(
       }
       std::set<std::string> const& languages =
         target->GetAllConfigCompileLanguages();
+      // cmGeneratorTarget::GetAllConfigCompileLanguages caches the target's
+      // sources. Clear it so that OBJECT library targets that are AUTOGEN
+      // initialized after this target get their added mocs_compilation.cpp
+      // source acknowledged by this target.
+      target->ClearSourcesCache();
       if (languages.count("CSharp")) {
         // Don't process target if it's a CSharp target
         continue;

Tests/QtAutogen/ObjectLibrary/CMakeLists.txt

@@ -16,3 +16,8 @@ target_compile_features(b PRIVATE ${QT_COMPILE_FEATURES})
 # Executable with OBJECT library generator expressions
 add_executable(someProgram main.cpp $<TARGET_OBJECTS:a> $<TARGET_OBJECTS:b>)
 target_link_libraries(someProgram ${QT_LIBRARIES})
+
+# Executable without its own AUTOMOC.
+add_executable(someProgram2 main.cpp)
+target_link_libraries(someProgram2 PRIVATE a b ${QT_LIBRARIES})
+set_property(TARGET someProgram2 PROPERTY AUTOMOC OFF)

Tests/RunCMake/ObjectLibrary/OwnSources-stderr.txt

@@ -10,4 +10,10 @@ CMake Error at OwnSources.cmake:[0-9]+ \(add_library\):
 Call Stack \(most recent call first\):
   CMakeLists.txt:[0-9]+ \(include\)
 +
+CMake Error at OwnSources.cmake:[0-9]+ \(add_library\):
+  The SOURCES of "A" use a generator expression that depends on the SOURCES
+  themselves.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)
++
 CMake Generate step failed\.  Build files cannot be regenerated correctly\.$