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+A trait was implemented on another which already automatically implemented it.
+
+Erroneous code examples:
+
+```compile_fail,E0371
+trait Foo { fn foo(&self) { } }
+trait Bar: Foo { }
+trait Baz: Bar { }
+
+impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
+impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
+impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
+impl Baz for Bar { } // Note: This is OK
+```
+
+When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
+definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
+`Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
+definition, so it is not useful to do this.