LYR-SEM0078

a conformance or parent list entry that does not fit

error · spec 05-interfaces

A :: list of a type, an extend block or an interface names something that is not an interface, repeats an entry, or makes an interface inherit itself; an associated type's or a type parameter's bound names a non-interface, or a type parameter repeats a bound.

Explanation

A conformance list says which interfaces a type satisfies, and a parent list which interfaces an interface extends; only interfaces stand there. Each entry is written once, and an interface's parent chain cannot lead back to itself. A type parameter's bounds are such a list too: <T :: [Display]> names an interface, once; From<int> and From<string> are two bounds.

Name an interface, remove the repeated entry, or break the cycle.

Wrong

struct Meters {
    value: float,
}

struct Point :: [Meters] {
    x: int,
}

fn main(): void {
    let _ = Point { x = 1 };
}

Right

struct Point {
    x: int,
}

fn main(): void {
    let _ = Point { x = 1 };
}