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.

Commencez par la reproduction liée dans TypeScript Playground et confirmez l’erreur signalée sur le retour générique à l’intérieur du case du switch. Suivez le narrowing du type générique du compilateur pour les clauses switch, puis ajoutez une couverture de régression montrant que le case réduit TagName et que le code attendu est vérifié par le système de types sans assertion.