Improve origin of constraints in subsumption checking

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index 169cf7b..1868ad0 100644 (file)
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -682,7 +682,13 @@ tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
        ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
 
                -- Deal with the dictionaries
-       ; co_fn1 <- instCall InstSigOrigin inst_tys (substTheta subst' theta)
+               -- The origin gives a helpful origin when we have
+               -- a function with type f :: Int -> forall a. Num a => ...
+               -- This way the (Num a) dictionary gets an OccurrenceOf f origin
+       ; let orig = case sub_ctxt of
+                       SubFun n -> OccurrenceOf n
+                       other    -> InstSigOrigin       -- Unhelpful
+       ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
        ; return (co_fn2 <.> co_fn1) }
 
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