diff options
| -rw-r--r-- | compiler/typecheck/TcInteract.hs | 5 |
1 files changed, 3 insertions, 2 deletions
diff --git a/compiler/typecheck/TcInteract.hs b/compiler/typecheck/TcInteract.hs index 47147d7a4d..180ddd39a4 100644 --- a/compiler/typecheck/TcInteract.hs +++ b/compiler/typecheck/TcInteract.hs @@ -602,12 +602,13 @@ we keep? More subtle than you might think! See Note [Solving superclass constraints] in TcInstDcls - Keep the one that has a non-trivial evidence binding. - Note [Tracking redundant constraints] again. Example: f :: (Eq a, Ord a) => blah - then we may find [G] sc_sel (d1::Ord a) :: Eq a + then we may find [G] d3 :: Eq a [G] d2 :: Eq a + with bindings d3 = sc_sel (d1::Ord a) We want to discard d2 in favour of the superclass selection from the Ord dictionary. + Why? See Note [Tracking redundant constraints] in TcSimplify again. * Finally, when there is still a choice, use IRKeep rather than IRReplace, to avoid unnecessary munging of the inert set. |