Fix type inference snafu with synom map
diff --git a/src/ty.rs b/src/ty.rs
index 1bb710f..e767852 100644
--- a/src/ty.rs
+++ b/src/ty.rs
@@ -556,7 +556,7 @@
             })
         )
         |
-        map!(syn!(Self_), |s: Self_| (None, s.into()))
+        map!(syn!(Self_), |s| (None, s.into()))
     ));
 
     impl Synom for ParenthesizedParameterData {