Non-unit types as union discriminants

TypeScript 3.2 makes narrowing easier by relaxing rules for what it considers a discriminant property.Common properties of unions are now considered discriminants as long as they contain some singleton type (e.g. a string literal, null, or undefined), and they contain no generics.

As a result, TypeScript 3.2 considers the error property in the following example to be a discriminant, whereas before it wouldn’t since Error isn’t a singleton type.Thanks to this, narrowing works correctly in the body of the unwrap function.

type Result<T> =
    | { error: Error; data: null }
    | { error: null; data: T };
function unwrap<T>(result: Result<T>) {
    if (result.error) {
        // Here 'error' is non-null
        throw result.error;
    }
    // Now 'data' is non-null
    return result.data;
}