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Diffstat (limited to 'compiler/GHC/Tc/Gen/Pat.hs')
| -rw-r--r-- | compiler/GHC/Tc/Gen/Pat.hs | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/compiler/GHC/Tc/Gen/Pat.hs b/compiler/GHC/Tc/Gen/Pat.hs index dba5bf5874..a1004e07c6 100644 --- a/compiler/GHC/Tc/Gen/Pat.hs +++ b/compiler/GHC/Tc/Gen/Pat.hs @@ -838,6 +838,15 @@ between alternatives. RIP GADT refinement: refinements have been replaced by the use of explicit equality constraints that are used in conjunction with implication constraints to express the local scope of GADT refinements. + +Note [Freshen existentials] +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is essential that these existentials are freshened. +Otherwise, if we have something like + case (a :: Ex, b :: Ex) of (MkEx ..., MkEx ...) -> ... +we'll give both unpacked existential variables the +same name, leading to shadowing. + -} -- Running example: @@ -888,6 +897,7 @@ tcDataConPat penv (L con_span con_name) data_con pat_ty_scaled ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX tenv ex_tvs -- Get location from monad, not from ex_tvs + -- This freshens: See Note [Freshen existentials] ; let -- pat_ty' = mkTyConApp tycon ctxt_res_tys -- pat_ty' is type of the actual constructor application @@ -974,6 +984,8 @@ tcPatSynPat penv (L con_span _) pat_syn pat_ty arg_pats thing_inside ; let all_arg_tys = ty : prov_theta ++ (map scaledThing arg_tys) ; checkExistentials ex_tvs all_arg_tys penv ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX subst ex_tvs + -- This freshens: Note [Freshen existentials] + ; let ty' = substTy tenv ty arg_tys' = substScaledTys tenv arg_tys pat_mult = scaledMult pat_ty |