From: simonpj@microsoft.com <unknown>
Date: Mon, 11 Dec 2006 16:05:57 +0000 (+0000)
Subject: Improve origin of constraints in subsumption checking
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=85de95cd55d5dcda29630b665b50c7575df6dddd

Improve origin of constraints in subsumption checking
---

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index 169cf7b..1868ad0 100644
--- 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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