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This regression prevents the generic-lens library from building. Here is a minimized test case:

{-# 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

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
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