From type/IO/Path

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method d(IO::Path:D: --> Bool:D)

Returns True if the invocant is a path that exists and is a directory. The method will fail with X::IO::DoesNotExist if the path points to a non-existent filesystem entity.

From type/IO/Special

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method d(IO::Special:D: --> False)

The 'directory' file test operator, always returns False.