Not narrowing nested discriminated union

[jonhue]

Bug Report

🔎 Search Terms

nested discriminated union

This issue is definitely related to #18758, but it is different from that issue in that the tag is also present in the top-level type.
As in the top-level type, the nested types form a generic union, I believe this is also related to #13995.

🕗 Version & Regression Information

This is the behavior in every version I tried.

⏯ Playground Link

Playground link with relevant code

💻 Code

enum Kind {
	A,
	B,
}

type InnerA = {
	kind: typeof Kind.A
}
type InnerB = {
	kind: typeof Kind.B
}

type Inner = InnerA | InnerB

type Outer<T extends Inner = Inner> = {
	kind: T['kind']
	inner: T
}

const handleInnerA = (inner: InnerA) => {}
const handleInnerB = (inner: InnerB) => {}

const fn = (outer: Outer) => {
	switch (outer.kind) {
		case Kind.A:
			return handleInnerA(outer.inner)
		case Kind.B:
			return handleInnerB(outer.inner)
	}
}

🙁 Actual behavior

The above code should compile without errors.

🙂 Expected behavior

The type of the inner property on outer should be narrowed based on the value of outer's kind property.

[RyanCavanaugh]

This is a correct error. Narrowing here would be unsound because this is a legal call:

const p: Outer = { kind: Kind.A, inner: { kind: Kind.B } };
fn(p);

See also #27808

[jonhue]

oh yes, thanks! you're absolutely right