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+The `+` type operator was used in an ambiguous context.
+
+Erroneous code example:
+
+```compile_fail,E0178
+trait Foo {}
+
+struct Bar<'a> {
+    x: &'a Foo + 'a,     // error!
+    y: &'a mut Foo + 'a, // error!
+    z: fn() -> Foo + 'a, // error!
+}
+```
+
+In types, the `+` type operator has low precedence, so it is often necessary
+to use parentheses:
+
+```
+trait Foo {}
+
+struct Bar<'a> {
+    x: &'a (Foo + 'a),     // ok!
+    y: &'a mut (Foo + 'a), // ok!
+    z: fn() -> (Foo + 'a), // ok!
+}
+```
+
+More details can be found in [RFC 438].
+
+[RFC 438]: https://github.com/rust-lang/rfcs/pull/438