Cryptic error message without --noImplicitAny

TypeScript Version: 4.1.0-dev.20201015

Search Terms: noImplicitAny

In the JS mode given the class inheritance hierarchy A -> B -> C, when B code refers to class declaration (e.g. in a clone() method) and the class is assigned to an object property (NS.B = class ...), an attempt to convert an instance of C to B returns a cryptic error message: Property <property name> is missing in type 'C' but required in type 'B'.

Expected behavior: This diagnostic isn't helping, maybe omit it?

Related Issues:

Code

class A {
	clone() {
		return new A();
	}
}

var NS = {};

NS.B = class extends A {
	constructor(v) {
		super();
		this.val = v;
	}

	//** @returns {NS.B} */  // this jsdoc comment fixes the error
	clone() {
		return new NS.B(this.val);
	}
}

class C extends NS.B {
	constructor() {
		super(1);
	}

	clone() {
		return new C();
	}
}


/** @returns {NS.B} */
function f() {
	return new C(); // error here: Property 'val' is missing in type 'C' but required in type 'B'.
}

{
  "compilerOptions": {
    "alwaysStrict": true,
    "esModuleInterop": true,
    "checkJs": true,
    "allowJs": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "Latest",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

Reproduce the example from the issue or its TypeScript Playground link with checkJs and the shown JavaScript class hierarchy. Start by tracing the type-checking path for the return in f() and the inferred type of NS.B. Done means the diagnostic is removed or made useful without requiring the JSDoc workaround, with the behavior covered by a regression test.