3.4.1 regression in distributed types

[aleclarson]

TypeScript Version: 3.5.0-dev.20190425

The example below was taken from a real world use-case and simplified.

The U type is distributed while the T type remains the original union.

The T type is wrapped with Box<T> when U is not an object.

type Box<T> = { value: T }
type Test<T> = [T] extends [infer U]
  ? U extends object
    ? U
    : U | Box<T>
  : never

type A = number | { foo: number }
type B = Test<A>

In theory, the B type should be identical to:

type B = number | { foo: number } | Box<number | { foo: number }>

But in the latest version, the B type is instead identical to:

type B = number | { foo: number } | Box<number | (number & { foo: number })>

This works correctly in 3.3.x but not 3.4.x and later.

Playground Link: click here

[rotu]

In between 4.1.5 and 4.2.3, the behavior changed from
type B = number | { foo: number } | Box<number | (number & { foo: number })>
to
type B = number | { foo: number } | Box<number & A>

I cannot say whether the new behavior is correct or not.