#12104: Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell into a hole" -------------------------------------+------------------------------------- Reporter: antalsz | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: TypeFamilies, | Operating System: MacOS X CustomTypeErrors | Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If I create a type family – open or closed – with a case that evaluates to a `TypeError`, and define a top-level binding with this type, loading the file with `-fdefer-type-errors` enabled (or via `:load!`/`:reload!`) panics GHC with "opt_univ fell into a hole". (And if I used `:load!` or `:reload!`, `-fdefer-type-errors` doesn't get unset.) A minimal example: {{{#!hs {-# LANGUAGE TypeFamilies, DataKinds, UndecidableInstances #-} import GHC.TypeLits type family F a where F a = TypeError (Text "error") err :: F () err = () }}} results in the panic {{{ ….hs:9:7: warning: [-Wdeferred-type-errors] • error • In the expression: () In an equation for ‘err’: err = () ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-apple-darwin): opt_univ fell into a hole {a4Va} }}} Adding more cases to the type family, or making it open, still cause the crash. This holds whether the error case is a final catch-all case, or something more like {{{#!hs type family F a where F () = TypeError (Text "error") F a = () }}} Just using a type synonym for `F` doesn't cause a panic, however, and nor does giving `err` the type `TypeError (Text "error")` directly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12104 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler