Improve RULE matching a bit more
Consider this example (provided by Roman)
foo :: Int -> Maybe Int -> Int
foo 0 (Just n) = n
foo m (Just n) = foo (m-n) (Just n)
SpecConstr sees this fragment:
case w_smT of wild_Xf [Just A] {
Data.Maybe.Nothing -> lvl_smf;
Data.Maybe.Just n_acT [Just S(L)] ->
case n_acT of wild1_ams [Just A] { GHC.Base.I# y_amr [Just L] ->
$wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf
}};
and correctly generates the rule
RULES: "SC:$wfoo1" [0] __forall {y_amr [Just L] :: GHC.Prim.Int#
sc_snn :: GHC.Prim.Int#}
$wfoo_smW sc_snn (Data.Maybe.Just @ GHC.Base.Int (GHC.Base.I# y_amr))
= $s$wfoo_sno y_amr sc_snn ;]
BUT we must ensure that this rule matches in the original function!
Note that the call to $wfoo is
$wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf
During matching we expand wild_Xf to (Just n_acT). But then we must also
expand n_acT to (I# y_amr). And we can only do that if we look up n_acT
in the in-scope set, because in wild_Xf's unfolding it won't have an unfolding
at all.
Happily, fixing the bug is easy: add a call to 'lookupRnInScope' in the
(Var v2) case of 'match'.