diff options
Diffstat (limited to 'compiler/GHC/Tc/Solver/InertSet.hs')
| -rw-r--r-- | compiler/GHC/Tc/Solver/InertSet.hs | 17 |
1 files changed, 11 insertions, 6 deletions
diff --git a/compiler/GHC/Tc/Solver/InertSet.hs b/compiler/GHC/Tc/Solver/InertSet.hs index c1f173ee14..e95a1debff 100644 --- a/compiler/GHC/Tc/Solver/InertSet.hs +++ b/compiler/GHC/Tc/Solver/InertSet.hs @@ -1558,23 +1558,28 @@ matchableGivens loc_w pred_w inerts@(IS { inert_cans = inert_cans }) matchable_given :: Ct -> Bool matchable_given ct | CtGiven { ctev_loc = loc_g, ctev_pred = pred_g } <- ctEvidence ct - = mightEqualLater inerts pred_g loc_g pred_w loc_w + = isJust $ mightEqualLater inerts pred_g loc_g pred_w loc_w | otherwise = False -mightEqualLater :: InertSet -> TcPredType -> CtLoc -> TcPredType -> CtLoc -> Bool +mightEqualLater :: InertSet -> TcPredType -> CtLoc -> TcPredType -> CtLoc -> Maybe Subst -- See Note [What might equal later?] -- Used to implement logic in Note [Instance and Given overlap] in GHC.Tc.Solver.Interact mightEqualLater inert_set given_pred given_loc wanted_pred wanted_loc | prohibitedSuperClassSolve given_loc wanted_loc - = False + = Nothing | otherwise = case tcUnifyTysFG bind_fun [flattened_given] [flattened_wanted] of - SurelyApart -> False -- types that are surely apart do not equal later - MaybeApart MARInfinite _ -> False -- see Example 7 in the Note. - _ -> True + Unifiable subst + -> Just subst + MaybeApart reason subst + | MARInfinite <- reason -- see Example 7 in the Note. + -> Nothing + | otherwise + -> Just subst + SurelyApart -> Nothing where in_scope = mkInScopeSet $ tyCoVarsOfTypes [given_pred, wanted_pred] |