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diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 0e1e5b0..4cf93e8 100644 (file)
--- 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