LYR-SEM0121

a second member of one name on a type

error · spec 05-interfaces

A type would hold two members of one name that a call could not tell apart: an extension beside the type's own member at a count it takes, two extensions of one name at one count, an extension beside a conformance's implementation or an interface default, two delegations or blanket blocks giving the name, or a static constant whose name the type already has.

Explanation

A type holds one member of a name and a count: T.name and x.name(…) have one answer, and nothing ranks one source over another. An extend block adds a name the type does not have, or the name at a count the type does not take — the body and the inherent blocks are one scope, overloaded by count as any scope is; it does not replace a member. The same holds for names that arrive through conformance blocks, interface defaults, delegations (by field) and blanket blocks.

Rename the extension, or make it the member itself; where a conformance implements the name, drop the extension or make it the conformance's implementation.

Wrong

struct V {
    x: int,
    fn tag(): int { return 1; }
}

extend V {
    fn tag(): int { return 2; }
}

fn main(): void {
    let _ = V { x = 0 }.tag();
}

Right

struct V {
    x: int,
    fn tag(): int { return 1; }
}

extend V {
    fn label(): int { return 2; }
}

fn main(): void {
    let _ = V { x = 0 }.tag() + V { x = 0 }.label();
}