Generic type unnarrowed by `switch...case`

Bug Report

🔎 Search Terms

• narrowing of generic types
• exhaustive switch case

🕗 Version & Regression Information

Theoretically this should be fixed by:

• https://github.com/microsoft/TypeScript/issues/13995
• https://github.com/microsoft/TypeScript/pull/43183

Similar issue:

• https://github.com/microsoft/TypeScript/issues/20375

I’m trying against the latest beta: 4.3.0-pr-43183-11 / 15fae38b39a370f1f597606e552dd40998a1ac49.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface TopLevelElementMap {
	"a": HTMLElement;
	"b": HTMLElement;
}

interface ElementAttributes {}

interface ElementAttributesMap {
	"a": ElementAttributes;
	"b": ElementAttributes;
}

class Element<TagName extends keyof TopLevelElementMap> {
	protected attributes: ElementAttributesMap[TagName] = {};

	public constructor(type: TagName) {}
}

const ElementTagNameMap = {
	"a": function(): Element<"a"> {
		return new Element("a");
	},
	"b": function(): Element<"b"> {
		return new Element("b");
	}
};

function createPrimitive<TagName extends keyof typeof ElementTagNameMap>(tagName: TagName) {
	switch (tagName) {
		case "a":
			return function(): Element<TagName> {
				//
				// [?] Shouldn't `tagName` get narrowed to just "a"?
				//
				return ElementTagNameMap[tagName]();
			};

		case "b":

		default:
			throw new Error("Unrecognized element `" + tagName + "`.");
	}
}

const a = createPrimitive("a");
const b = createPrimitive("b");

As a workaround I can assert as Element<TagName> on the inner-most return statement to silence the error.

🙁 Actual behavior

Type 'Element<"a"> | Element<"b">' is not assignable to type 'Element<TagName>'.
  Type 'Element<"a">' is not assignable to type 'Element<TagName>'.
    Type '"a"' is not assignable to type 'TagName'.
      '"a"' is assignable to the constraint of type 'TagName', but 'TagName' could be instantiated with a different subtype of constraint '"a" | "b"'. (2322)

🙂 Expected behavior

TagName should be narrowed to one of the possible values dictated by the case clause.

[mkantor]

Is there a more targeted issue for just this problem?

• e[x] when x is a type parameter constrained to K1 | K2 and K1 and K2 are both valid keys of e should be legally narrowed via a contextual type

In case it helps, here’s a simpler situation that demonstrates it without the other problems mentioned by @RyanCavanaugh:

const stuff = {
    string: '',
    number: 0,
}
function f<T extends 'string' | 'number'>(x: T): void {
    if (x === 'string') {
        stuff[x].length // error :(
    }
}

[ouuan]

I found a case that could be relevant: Playground link.

I’ve confirmed that in this piece of code, const a: "a" = x, takesA(x) and const never: never = x have errors in TypeScript v4.2.3. But in v4.3.5, only obj[x] have an error. This could be the result of #13995 being partially fixed.