[GHC] #14339: GHC 8.2.1 regression when combining GND with TypeError (solveDerivEqns: probable loop)

[GHC]

#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>
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