#14339: GHC 8.2.1 regression when combining GND with TypeError (solveDerivEqns: probable loop) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: deriving, | Operating System: Unknown/Multiple CustomTypeErrors | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This code panics on GHC 8.2.1 and later: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE UndecidableInstances #-} module Bug where import GHC.TypeLits newtype Baz = Baz Foo deriving Bar newtype Foo = Foo Int class Bar a where bar :: a instance (TypeError (Text "Boo")) => Bar Foo where bar = undefined }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) ghc: panic! (the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-linux): solveDerivEqns: probable loop DerivSpec ds_loc = Bug.hs:9:12-14 ds_name = $fBarBaz ds_tvs = [] ds_cls = Bar ds_tys = [Baz] ds_theta = [ThetaOrigin to_tvs = [] to_givens = [] to_wanted_origins = [Bar Foo, (Foo :: *) ~R# (Baz :: *)]] ds_mechanism = newtype [[s0_a1D7[fuv:0]]] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcDerivInfer.hs:515:9 in ghc:TcDerivInfer }}} This is a regression since GHC 8.0.2, in which it does compile successfully. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14339 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler