Support open-ended unions

Suggestion

Ability to discriminate between union members when not all union members are known.

Use Cases

• redux actions: See https://github.com/Microsoft/TypeScript/issues/2214#issuecomment-352103680
• This would enable us to discriminate on Node kinds without converting Node to a huge union (which slows down compilation too much). (Also, Node is effectively open-ended because we add new kinds fairly often.)

Examples

interface Shape {
    unique kind: string;
}

interface Square extends Shape {
    kind: "square";
    size: number;
}

interface Circle extends Shape {
    kind: "circle";
    radius: number;
}

// other shapes may exist

function area(s: Shape) {
    switch (s.kind) {
        case "square":
            return s.size * s.size;
        case "circle":
            return Math.PI * s.radius ** 2;
        default:
            return 0; // Or hand off to some other function that handles other Shape kinds
    }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

Workarounds

Cast the general type to a union of known type.

function area(sIn: Shape) {
    const s = sIn as Square | Circle;
    ...

Start by reviewing the open-ended union examples and the linked Redux-actions discussion to understand the intended narrowing behavior. Done means known discriminant cases narrow to their corresponding members while unknown cases remain valid and reach the fallback; the issue does not name implementation files or tests.