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.

リンクされた TypeScript Playground の再現コードから始め、switch の case 内のジェネリックな return で報告されたエラーを確認します。switch 節に対するコンパイラのジェネリック型の絞り込みを追跡し、case によって TagName が絞り込まれ、期待されるコードが assertion なしで型チェックを通過することを示すリグレッションテストを追加します。