Cannot add library build directory to include_directories

[mardy]

Describe the bug

Meson 0.6.3.

I'm trying to build a static library including all the moc files that are needed in our test suite (the reason is that, otherwise, meson generates thousands of build targets, since these moc files are needed by every test binary):

moc_library = static_library('all_mocs',
    qt5.compile_moc(sources: all_sources, headers: all_headers),                      
    dependencies: test_core_dependencies,
    include_directories: include_directories('.'),                                                    
)                                                                                                     
moc_library_dep = declare_dependency(                                                                 
    include_directories: include_directories('.'),
    link_with: moc_library,                                                                           
)                                                                                                     

The problem is that the files generated by moc are written into a liball_mocs.a.p subdirectory (of tests/Core), which is not added to the include path. So, when the main target files are built, they cannot find the moc-generated files:

# (command edited with line breaks for readability)
c++ -Itests/Core/test_task_launcher.p -Itests/Core -I../tests/Core -Isrc/common \
    -I../src/common -Isrc/service -I../src/service \
    -I/usr/include/qt5/QtDBus -I/usr/include/qt5 -I/usr/include/qt5/QtCore \
    -I/usr/include/qt5/QtNetwork -I/usr/include/qt5/QtTest \
    -DUNIT_TESTS -MD -MQ \
    tests/Core/test_task_launcher.p/_home_mardy_src_git_runtime-manager_src_service_Core_task_launcher.cpp.o \
    -MF tests/Core/test_task_launcher.p/_home_mardy_src_git_runtime-manager_src_service_Core_task_launcher.cpp.o.d \
    -o tests/Core/test_task_launcher.p/_home_mardy_src_git_runtime-manager_src_service_Core_task_launcher.cpp.o \
    -c /home/mardy/src/git/runtime-manager/src/service/Core/task_launcher.cpp
/home/mardy/src/git/runtime-manager/src/service/Core/task_launcher.cpp:340:10: fatal error: task_launcher.moc: No such file or directory
  340 | #include "task_launcher.moc"
      |          ^~~~~~~~~~~~~~~~~~~
compilation terminated.

If I use include_directories: include_directories('./liball_mocs.a.p'), then following builds do work, but the first clean build won't, because meson complains that the directory does not exist. It's not clear to me why include_directories('.') does not work in this case.