X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=4cf93e83da60217c0249cc28a1b3a00745fa5caf;hb=8c554937f8824da81e03e504936320b3321022ed;hp=0e1e5b0077270ae32d9de49d260c4f28dbb0b7ec;hpb=e5a8d57c85d42007c8cc561e6d6b805c23603fc0;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 0e1e5b0..4cf93e8 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -710,6 +710,12 @@ tcSimplifyInfer doc tau_tvs wanted
 		-- irreds2 will be empty.  But we don't want to generalise over b!
 	; let preds2 = fdPredsOfInsts irreds2	-- irreds2 is zonked
 	      qtvs   = growInstsTyVars irreds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
+		---------------------------------------------------
+		-- BUG WARNING: there's a nasty bug lurking here
+		-- fdPredsOfInsts may return preds that mention variables quantified in
+		-- one of the implication constraints in irreds2; and that is clearly wrong:
+		-- we might quantify over too many variables through accidental capture
+		---------------------------------------------------
 	; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
 	; extendLIEs free