Nested unions (and intersections) are eagerly flattened #42075

TypeScript Version: 4.1.3

Search Terms: nested union, flatten union

Code

type T = number | string
let x: T | null // Inspect the type of x

Expected behavior:

Since in most cases, TypeScript takes care to preserve type names, I'd hope x to get type T | null.

Actual behavior:

Union types that are part of another union type are eagerly flattened into the outer type, producing a type printed as number | string | null.

I am running into this when using the TypeScript compiler as a library as part of a documentation generating system. I have some named union types in my API that become noisy and hard to interpret for users when they are expanded every time they appear in some union type.

Playground Link: here

Related Issues: No.