`declare_dependency()` `sources` inconsistently added to include paths

[pobrn]

Consider

# directory 'a'
some_generated_file = custom_target(
        'some_generated_file.h',
        output: 'some_generated_file.h',
        ...
)

# top-level
x = declare_dependency(
        sources: [
                some_generated_file,
                'b/some_static_file.h',
        ],
)

If another target depends on x, then -Ia (location of some_generated_file.h) will be added implicitly. However, -Ib, parent directory of some_static_file.h, listed in sources as well, will not be added.

Furthermore, setting implicit_include_directories: false will cause -Ia to not be added.

What is the reason for this inconsistency? Is there a way to specify a generated header file in declare_dependency() in such a way that its parent directory won't be added implicitly? (In my case the file is already generated into a directory added explicitly using include_diretories, and I don't want to be able to do #include "some_generated_file.h", only #include <some/path/some_generated_file.h should work. Or is there an issue with this?)