#15142: GHC HEAD regression: tcTyVarDetails -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.6.1 Component: Compiler | Version: 8.5 (Type checker) | Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This regression prevents the `generic-lens` library from building. Here is a minimized test case: {{{#!hs {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind class ListTuple (tuple :: Type) (as :: [(k, Type)]) where type ListToTuple as :: Type }}} On GHC 8.4.2, this compiles, but on GHC HEAD, it panics: {{{ $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs[1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.5.20180511 for x86_64-unknown-linux): tcTyVarDetails co_aWx :: (k_aWt[tau:2] :: *) ~# (* :: *) Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1162:37 in ghc:Outputable pprPanic, called at compiler/basicTypes/Var.hs:497:22 in ghc:Var }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15142 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler