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.

从链接的 TypeScript Playground 复现开始，确认 switch case 内泛型返回值上报告的错误。跟踪编译器针对 switch 子句的泛型类型收窄，然后添加回归测试覆盖，表明 case 会收窄 TagName，并且预期代码无需 assertion 即可通过类型检查。