Duplicate base directories with file sets

I just discovered by accident that target_sources() does not complain if you add the same BASE_DIR to a file set more than once. It appears this works fine with no side effects, as far as I could tell. It turns out this is quite useful, since it allows the following pattern to be used in both in-source and out-of-source builds without any special handling:

    EXPORT_FILE_NAME include/Something/something_export.h


In practice, a project might move the generate_export_header() parts down into a subdirectory, but I’ve put it all in the one file above just to more easily demonstrate the main relevant aspects of the pattern.

Given that it is a useful behavior to allow a base directory to be added more than once and that it is already in a release, should we formally document it as something projects can rely on? Seems we might as well.

CC: @kyle.edwards

Is it actually stored twice (having been in the code, I would guess “yes”)?

That said, I don’t see anything in the current wording suggesting that this would not work at least as a directory is not a subdirectory of itself.