🔎 Search Terms

type shape equivalency getters setters

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type shape equivalency

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEAi0G8BQ1XTALmgOwK4FsAjAUwCcBuFNQrCAF1IEtsBzStaYAe23tN2B0upABQBKKqmQcOdABaMIAOjDQAvNACMAJgDM7GanmKlhddABEAQQBCAYQsHUAXySukoSDACiiSdAAHJgA3MDpiaAB9ABMsWCdOHj4BIVFYuAkOaUNoY2UY82iE1w4WYjp0cURSctxSbFyFfOiVchK0MorCKoQaujqGvKUY0za3JCQAM1xsQUYeaEnsEXTYMWgsYK5GaKRs1BbVDV0T8lQAenPofmw6RnwIybBGEDqIkmAwXAgI+TJiADkMGwXGgPzo4VI4yQIHK0GIWF8GmwxAA7tBvCIUejYOIxJQpstiPiLlcIHIuLgQNFrsQAsIKr4QRVuPgAmFGIRYdBUYx5HBoFjQeDIRAJEA

💻 Code

class D {
    a: number;
    b: string;
    constructor()
    {
        this.a = 123;
        this.b = "ABC";
    }
}

class E {
    private _d: D;
    constructor(d: D)
    {
        this._d = d;
    }
    get a() {return this._d.a;}
    get b() {return this._d.b;}
}

function fn(d: D) : void
{
    d.a = 333;  // runtime failure because there's no setter
}

let e: E = new E(new D());

fn(e);  // should report E not compatible with D (no setters)

🙁 Actual behavior

No errors are reported when passing and instance of E in a call where D is expected because E has getters for all properties of D, even though only getters exist and there are no setters. As a result of this, a runtime error will be reported if an attempt to assign any of those properties is made.

🙂 Expected behavior

The type checker should consider type shapes equivalent only if they have same get/set behavior. In the example above, if a and b in D were readonly, OR if E had get/set for both properties, then the example would contain no errors. Both of these conditions should be checked when evaluating type shape equivalency.

Additional information about the issue

TypeScript 5.5.3