containers version used in GHC 9.0 is missing several important bug fixes
The ghc-9.0
branch currently uses containers-0.6.2.1
, which has several important correctness issues that were fixed in v0.6.3.1: http://hackage.haskell.org/package/containers-0.6.3.1/changelog.
I suggest that GHC 9.0 include the latest containers-0.6.4.1
instead.