Why pg_dump overwrites dump file?

I noticed this problem while testing the other patch.

When I do custom-format dump, if a target file exists, pg_dump will just go ahead overwrite the existing file; however, when I do directory dump, if a target dir exists, pg_dump will fail with an error “directory xxx is not empty”.

Behaviors of the two types of pg_dump are inconsistent, I wonder if that’s by design?

Best regards,