Error code E0178

The + type operator was used in an ambiguous context.

Erroneous code example:

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.