[project @ 2003-02-20 16:02:57 by simonpj]

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diff --git a/ghc/compiler/typecheck/TcInstDcls.lhs b/ghc/compiler/typecheck/TcInstDcls.lhs
index cf705ae..d312cee 100644 (file)
--- 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'