nested-sum-types.ts

interface Inner<T> {
    value: T;
}

interface Numeric {
    content: Inner<number>;
}

interface Text {
    content: Inner<string>;
}

type Expr = Text | Numeric;

function evaluate<T>(content: Inner<T>): T {
    return content.value;
}

// compiles
function ok(expr: Expr) {
    const content: Inner<string|number> = expr.content;
    evaluate(content);
}

// doesn't
function err(expr: Expr) {
    evaluate(expr.content);
}