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

Riproduci l’esempio dell’issue o il relativo link a TypeScript Playground con checkJs e la gerarchia di classi JavaScript mostrata. Inizia tracciando il percorso del controllo dei tipi per il valore restituito in f() e il tipo inferito di NS.B. Il lavoro è completo quando la diagnostica viene rimossa o resa utile senza richiedere il workaround JSDoc e il comportamento è coperto da un test di regressione.