Buglet in GHC.Tc.Module.checkBootTyCon

GHC.Tc.Module.checkBootTyCon` has this code:

checkBootTyCon :: Bool -> TyCon -> TyCon -> Maybe SDoc
checkBootTyCon is_boot tc1 tc2
  ...
  | Just c1 <- tyConClass_maybe tc1
  ...
  , Just env <- eqVarBndrs emptyRnEnv2 clas_tvs1 clas_tvs2
  = let
       eqSig (id1, def_meth1) (id2, def_meth2)
         = ...
           if is_boot
               then check (liftEq eqDM def_meth1 def_meth2)
                          (text "The default methods associated with" <+> pname1 <+>
                           text "are different")
               else check (subDM op_ty1 def_meth1 def_meth2)
                          (text "The default methods associated with" <+> pname1 <+>
                           text "are not compatible")
       ...

       subDM t1 (Just (_, GenericDM t2)) (Just (_, VanillaDM))
        = eqTypeX env t1 t2            <----------------------------- Yikes!
       subDM _ (Just (_, GenericDM t1)) (Just (_, GenericDM t2))
        = eqTypeX env t1 t2

The line labelled Yikes! is totally wrong. Here t1 and t2 both come from tc1 and share a common name space. We should not rename t2 with tc2's renaming env!

The the fix is easy: replace eqTypeX env t1 t2 on that line with eqType t1 t2. It would be better to rename the (confusingly numbered) t2 since it comes from tc1.

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It is difficult to trigger this bug. It was revealed when working on !9343 (closed)