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.