#7938: Unbound kind variable can appear in RHS of associated type -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHC accepts | Test Case: indexed- invalid program | types/should_fail/T7938 Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- Comment (by Ryan Scott ): In [changeset:"0829821a6b886788a3ba6989e57e25a037bb6d05/ghc" 0829821a/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="0829821a6b886788a3ba6989e57e25a037bb6d05" Implicitly bind kind variables in type family instance RHSes when it's sensible Summary: Before, there was a discrepancy in how GHC renamed type synonyms as opposed to type family instances. That is, GHC would accept definitions like this one: ```lang=haskell type T = (Nothing :: Maybe a) ``` However, it would not accept a very similar type family instance: ```lang=haskell type family T :: Maybe a type instance T = (Nothing :: Maybe a) ``` The primary goal of this patch is to bring the renaming of type family instances up to par with that of type synonyms, causing the latter definition to be accepted, and fixing #14131. In particular, we now allow kind variables on the right-hand sides of type (and data) family instances to be //implicitly// bound by LHS type (or kind) patterns (as opposed to type variables, which must always be explicitly bound by LHS type patterns only). As a consequence, this allows programs reported in #7938 and #9574 to typecheck, whereas before they would have been rejected. Implementation-wise, there isn't much trickery involved in making this happen. We simply need to bind additional kind variables from the RHS of a type family in the right place (in particular, see `RnSource.rnFamInstEqn`, which has undergone a minor facelift). While doing this has the upside of fixing #14131, it also made it easier to trigger #13985, so I decided to fix that while I was in town. This was accomplished by a careful blast of `reportFloatingKvs` in `tcFamTyPats`. Test Plan: ./validate Reviewers: simonpj, goldfire, austin, bgamari Reviewed By: simonpj Subscribers: rwbarton, thomie GHC Trac Issues: #13985, #14131 Differential Revision: https://phabricator.haskell.org/D3872 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7938#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler