Preserve union type name within larger union

[ghost]

TypeScript Version: 3.1.0-dev.20180731

Code

type Value =
    | { kind: "number", n: number }
    | { kind: "string", s: string };
function f(value: Value | undefined): void {}
f(undefined);

Expected behavior:

Quick info on f shows function f(value: Value | undefined): void

Actual behavior:

Quick info shows:

function f(value: {
    kind: "number";
    n: number;
} | {
    kind: "string";
    s: string;
} | undefined): void

Related Issues: #14662

[samijaber]

Reduced test-case without functions or tagged unions:

type Foo = 'a' | 'b'
declare const a: Foo;
declare const b: Foo;

const value = a // Foo (as expected)

const values1 = [a, b] // Foo[] (as expected)

const values2 = [a, 1] // (number | "a" | "b")[], but expected (number | Foo)[]

[OliverJAsh]

Will be fixed by #42149?