Add regression test for issue 283

    error[E0276]: impl has stricter requirements than trait
        --> tests/test.rs:1699:5
         |
    1692 |         async fn a();
         |         ------------- definition of `a` from trait
    ...
    1699 |     #[async_trait]
         |     ^^^^^^^^^^^^^^ impl has extra requirement `T: 'async_trait`
         |
         = note: this error originates in the attribute macro `async_trait` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/test.rs

@@ -1683,3 +1683,23 @@ pub mod issue281 {
         }
     }
 }
+
+pub mod issue283 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Trait {
+        async fn a();
+    }
+
+    pub trait Bound {
+        fn b();
+    }
+
+    #[async_trait]
+    impl<T: Bound> Trait for T {
+        async fn a() {
+            Self::b();
+        }
+    }
+}