Generic mapped type constrained by a type parameter has different members / index signatures when constraint of that type parameter is a union #27819

function foo1<K extends string>(arg: { [P in K]: string }) {
    // Actual: Error: Element implicitly has an 'any' type because type '{ [P in K]: string; }' has no index signature.
    return arg["test"];
}
function foo2<K extends string | number>(arg: { [P in K]: string }) {
    // Actual: No error
    return arg["test"];
}
function foo3<K extends "foo">(arg: { [P in K]: string }) {
    // Actual: Error: Property 'foo' does not exist on type '{ [P in K]: string; }'.
    return arg.foo;
}
function foo4<K extends "foo" | "bar">(arg: { [P in K]: string }) {
    // Actual: No error
    return arg.foo;
}