Some user code likes to assign to _v_attrs and/or write into the dictionary to piggyback on caching. Since ImmutableDeclaration provides no way to clear _v_attrs (it assumes it stays None), this can lead to incorrect results.

This is not a regression in 5.0 but a pre-existing problem.

Proposal: Make ImmutableDeclaration use a property to always return a fresh dictionary for _v_attrs.