# Error code E0178

The `+` type operator was used in an ambiguous context.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    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:

    #![allow(unused)]
    fn main() {
    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.