Fix handling of closed type families in Backpack.
Summary: A few related problems: - CoAxioms, like DFuns, are implicit and never exported, so we have to make sure we treat them the same way as DFuns: in RnModIface we need to rename references to them with rnIfaceImplicit and in mergeSignatures we need to NOT check them directly for compatibility (the test on the type family will do this check for us.) - But actually, we weren't checking if the axioms WERE consistent. This is because we were forwarding all embedded CoAxiom references in the type family TyThing to the merged version, but that reference was what checkBootDeclM was using as a comparison point. This is similar to a problem we saw with DFuns. To fix this, I refactored the handling of implicit entities in TcIface for Backpack. See Note [The implicit TypeEnv] for the gory details. Instead of passing the TypeEnv around explicitly, we stuffed it in IfLclEnv. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: bgamari, simonpj, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2928
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