GNU ar has a P modifier that changes filename comparisons to use full paths instead of the basename. As noted in the GNU docs, regular archives are not created with full path names, so P is used to deal with archives created by other archive programs (e.g. see the updated absolute-paths.test test case).
Since thin archives use full path names -- paths are relative to the archive -- it seems very error prone to not imply P when dealing with thin archives, so P is implied in those cases. (I think this is a deviation from GNU ar that makes sense).
This fixes PR37436 via https://github.com/ClangBuiltLinux/linux/issues/33.