#13985: GHC 8.0 regression: ‘k’ is not in scope during type checking, but it passed the renamer -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: #13738 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I tried looking at this, but I'm not nearly smart enough to sort out the chicken-and-egg problem that I uncovered. I //think// the solution would be to call [http://git.haskell.org/ghc.git/blob/d774b4e2de4f07d2432b67010305fede7aeefc78... reportFloatingKvs] somewhere in the code that typechecks data family instances (e.g., [http://git.haskell.org/ghc.git/blob/d774b4e2de4f07d2432b67010305fede7aeefc78... tcDataFamInstDecl] seems like a good candidate). However, there's a serious problem with that: `reportFloatingKvs` needs tyvars to do its thing, but `tcDataFamInstDecl` starts out only having access to type patterns, not tyvars. To obtain tyvars, `tcDataFamInstDecl` calls `tcFamTyPats`, which in turn calls `kcDataDefn` to kind-check the type patterns. But therein lies the problem: `kcDataDefn` kind-checks each of the data constructors! By the time we do this, it's too late, since `k` wasn't rejected yet, so when we hit `k` when kind-checking a data constructor's existential type variables, GHC can't find it and throws an internal error. That is, `reportFloatingKvs` needs tyvars that `tcFamTyPats` produces, but `tcFamTyPats` can't produce the tyvars because it hits a kind variable which should have been rejected earlier by `reportFloatingKvs`. Ugh. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13985#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler