X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcInstDcls.lhs;h=d312cee87f1a7954258e8f9c7265c2ce5b814b4c;hb=e83fa2eba7ef2302fae8a6be86a9f495b4144387;hp=cf705ae2beffdaaa3079460665a9af2223592d03;hpb=3194c5289207114a3417be884b2174a4375b808f;p=ghc-hetmet.git

diff --git a/ghc/compiler/typecheck/TcInstDcls.lhs b/ghc/compiler/typecheck/TcInstDcls.lhs
index cf705ae..d312cee 100644
--- a/ghc/compiler/typecheck/TcInstDcls.lhs
+++ b/ghc/compiler/typecheck/TcInstDcls.lhs
@@ -629,6 +629,18 @@ tcMethods clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
 	--
 	-- Solution: make meth_insts available, so that 'then' refers directly
 	-- 	     to the local 'bind' rather than going via the dictionary.
+	--
+	-- BUT WATCH OUT!  If the method type mentions the class variable, then
+	-- this optimisation is not right.  Consider
+	--	class C a where
+	--	  op :: Eq a => a
+	--
+	--	instance C Int where
+	--	  op = op
+	-- The occurrence of 'op' on the rhs gives rise to a constraint
+	--	op at Int
+	-- The trouble is that the 'meth_inst' for op, which is 'available', also
+	-- looks like 'op at Int'.  But they are not the same.
     let
 	all_insts      = avail_insts ++ catMaybes meth_insts
 	xtve	       = inst_tyvars `zip` inst_tyvars'