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The following file causes a panic in 9.2.1 and on the ghc-9.2 branch

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
module M where

type family Item l
type instance Item [a] = a

f :: Enum (Item l) => l
f = f

x = (_ f) :: Int

  (GHC version 9.2.0.20220224:
	mkCoCast
  (Enum (Sym (D:R:Item[][0] <Char>_N)))_R
  Enum Char ~ Enum (Item [Char])
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Coercion.hs:1675:5 in ghc:GHC.Core.Coercion

HEAD is good:

M.hs:10:6: error:
    • Found hole: _ :: t0 -> Int
      Where: ‘t0’ is an ambiguous type variable

I haven't checked which commit fixed it. There are no other tickets mentioning mkCoCast, so I think it's worth checking if the bug is actually fixed or merely hidden.

Credits to Dominic Steinitz for the initial report.