Panic when defining an instance for (~~) [ppr_equality: homogeneity]

In GHC 8.2.1 and HEAD, compiling this file panics:

import Data.Type.Equality
instance (~~) Int Int

$ /opt/ghc/8.2.1/bin/ghc Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )

Bug.hs:2:10: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.1 for x86_64-unknown-linux):
        ppr_equality: homogeneity
  ~~
  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/iface/IfaceType.hs:1008:31 in ghc:IfaceType

We should probably give a similar error as for when one attempts to define an instance for (~):

instance (~) Int Int

$ /opt/ghc/8.2.1/bin/ghc Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )

Bug.hs:1:10: error:
    • Illegal instance declaration for ‘Int ~ Int’
        Manual instances of this class are not permitted.
    • In the instance declaration for ‘(~) Int Int’
  |
1 | instance (~) Int Int
  |          ^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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