GHC 8.2.1 regression when combining GND with TypeError (solveDerivEqns: probable loop)

This code panics on GHC 8.2.1 and later:

{-# 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.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.4.1

changed weight to 7

added Tbug Trac import typechecker labels

This is technically my fault, since this regression first started happening in 639e702b (Refactor DeriveAnyClass's instance context inference). That being said, I don't have any inclination as to what part of that patch tickles this panic.

mentioned in commit 13fdca3d

Trac metadata

Trac field	Value
Test case	- → testsuite/tests/deriving/should_compile/T14339

added 1 deleted label

changed milestone to %8.2.3

This was marked as merge with no release target, so I'll optimistically assume you meant 8.2.3, if there is one planned.

changed milestone to %8.2.2

Ahh, good catch RyanGlScott. Actually I think we can sneak it in to 8.2.2.

closed

Merged in a05d71a7.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in issue #15052 (closed)

added custom type errors deriving labels

added compiler crash label

added Phigh label

mentioned in issue #22696 (closed)