Re: [GHC] #8913: either bug or confusing error message mixing PolyKinds and TypeFamilies

#8913: either bug or confusing error message mixing PolyKinds and TypeFamilies -------------------------------------+------------------------------------- Reporter: ghorn | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.1-rc2 checker) | Keywords: PolyKinds, Resolution: | TypeFamilies Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"6ae678e31a5fdd3b0bd1f8613fe164012bb630f4/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="6ae678e31a5fdd3b0bd1f8613fe164012bb630f4" Flattener preserves synonyms, rewriteEvidence can drop buggy "optimisation" There was a special case in rewriteEvidence, looking like: = return (Just (if ctEvPred old_ev `tcEqType` new_pred then old_ev else old_ev { ctev_pred = new_pred })) But this was wrong: old_pred and new_pred might differ in the kind of a TyVar occurrence, in which case tcEqType would not notice, but we really, really want new_pred. This caused Trac #8913. I solved this by dropping the whole test, and instead making the flattener preserve type synonyms. This was easy because TcEvidence has TcTyConAppCo which (unlike) Coercion, handles synonyms. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8913#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler