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
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

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

Yes please!

Trac metadata

Trac field	Value
Differential revisions	- → D4132

mentioned in commit 85aa1f42

closed

changed milestone to %8.4.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

Trac metadata

Trac field	Value
Test case	- → T14390

mentioned in commit 29ae8337

added compiler crash label

added Pnormal label