Cannot assign to ... because it is a read-only property when using type guard in ctor

A common pattern is declaring narrower types for members in derived classes. However, when combined with type guards, assignement to readonly members fails with an unexpected "Cannot assign to ... because it is a read-only property" error.

TypeScript Version: 3.7.2

Search Terms:
class member read-only property type narrowing covariant type guard

Code

class Foo {
    public readonly x: string | number;

    constructor() {
        if (isBar(this))
            // error TS2540: Cannot assign to 'x' because it is a read-only property.
            this.x = 'string';
        else
            // no error here
            this.x = 123;
    }
}

class Bar extends Foo {
    // Declare narrower type for member in derived class
    public readonly x: string;
}

function isBar(foo: Foo): foo is Bar {
    return foo instanceof Bar;
}

Expected behavior:

Assignment to this.x should be allowed with or without type guard in the constructor

Actual behavior:

error TS2540: Cannot assign to 'x' because it is a read-only property.

Playground Link:

https://www.typescriptlang.org/play/?ssl=2&ssc=1&pln=22&pc=1#code/FAYwNghgzlAEBiB7RsDexadgBwK4CMwBLEWAJwFMIATRAOzAE9YAPALligBcyi6BzWAB9YdXAFt8FMgG5gGLCHrcyuEF0RkAFAEo0CrFiIAzWFqJQAQhG1cAFhZ06Dh1wHo3saWU2wAKgDKAEwArAAsAAwcAMIQdHSIXLDQUET8dLAasADkLNmwUiAQuFAUsERJFsnkVNQAtPRMOD7Y0lyMAHQurlj2Fh0ssAC8sACMQQDMcj2YFGCl3TMeoijevnbSFIs9fVADw2OT01gAvsBnoJAwsNZkXixcFHTUcEgo6IZ4hCQ1NI3M7E4PD4-DkF2MuDo6iI9HKVhsWmMyA4bx0HCRKCqt30hkoXFwZAyGPKdG4cRAFEQpluYOAQA

Related Issues:

It is as if the type guard creates an invisible alias (cf https://github.com/microsoft/TypeScript/issues/14241), and asignment inside the type guard fails.

Commencez par reproduire l’exemple TypeScript 3.7.2 dans le Playground indiqué et comparez les affectations protégée et non protégée. Suivez les vérifications du compilateur concernant le garde de type et les propriétés readonly, puis ajoutez un test de régression pour l’exemple. C’est terminé lorsque les deux affectations dans le constructeur sont acceptées sans produire TS2540.