#13271: GHC Panic With Injective Type Families ----------------------------------------+--------------------------------- Reporter: wayofthepie | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: Unknown/Multiple | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+--------------------------------- The following causes GHC to panic: {{{ {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilyDependencies #-} module T where import GHC.TypeLits data T1 = T1 type T2 = TypeError (Text "You can't do that!") type family X i = r | r -> i where X 1 = T1 X 2 = T2 }}} {{{ $ ghc T.hs [1 of 1] Compiling T ( T.hs, T.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): isInjectiveTyCon sees a TcTyCon T2 }}} This may be related to https://ghc.haskell.org/trac/ghc/ticket/11560. Without injectivity it gives the expected type error with the message "You can't do that!" {{{ {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} module T where import GHC.TypeLits data T1 = T1 type T2 = TypeError (Text "You can't do that!") type family X i where X 1 = T1 X 2 = T2 }}} {{{ $ ghc T.hs [1 of 1] Compiling T ( T.hs, T.o ) T.hs:9:1: error: • You can't do that • In the type synonym declaration for ‘T2’ }}} This isn't something anyone would intentionally write, as it could never type check with the type synonym T2 being a TypeError, but a panic is a panic so I said I'd raise it anyway. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13271 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler