LYR-SEM0032

or-pattern alternatives that bind differently

error · spec 09-patterns

The alternatives of an or-pattern p | q do not bind the same names, or bind one name at different types.

Explanation

The arm after an or-pattern runs whichever alternative matched, so each name it uses must be bound by every alternative, and with one type.

Bind the same names in every alternative, or split the alternatives into separate arms.

Wrong

enum Pair { A(int), B(int) }

fn value(p: Pair): int {
    return match (p) { .A(x) | .B(y) => 1 };
}

fn main(): void {
    let _ = value(Pair.A(1));
}

Right

enum Pair { A(int), B(int) }

fn value(p: Pair): int {
    return match (p) { .A(_) | .B(_) => 1 };
}

fn main(): void {
    let _ = value(Pair.A(1));
}