Isolated declarations does not handle object getters and setters with different types

[Luca Casonato]

🔎 Search Terms

• isolated declarations
• object getter
• object setter

🕗 Version & Regression Information

5.5.0-dev.20240514

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99&isolatedDeclarations=true&ts=5.5.0-dev.20240514#code/KYDwDg9gTgLgBAYwgOwM7wIZwLxwN4BQcxcA5sPAEYAUAlAFxzpQCWyp+RJ3UFArlGRwARMK7EAvgBpxTCnBoA3DABtGzNqVqduEghIDcBAjACeYYHADKMVuwCCKlhlQ4mtzUdCRYiFOkQ3Qm5yeAATOkYbO1JHZ1dg7hJeGAEhUVlpWVR5COU1dxjtRMl9I2NvaHgkNHhLXBKyeQAzSMLNHSTiFLSRMV0Zbhz4VvzGZD4AW0pgKGLMsqA

💻 Code

export const a = {
    get b(): string {
        return ""
    },
    set b(val: string) {
    }
};

type StringAlias = string;
export const c = {
    get d(): StringAlias {
        return ""
    },
    set d(val: string) {
    }
};


export const e = {
    get f(): string {
        return ""
    },
    set f(val: number) {
    }
};

🙁 Actual behavior

TypeScript emits:

export declare const a: {
    b: string;
};
export declare const c: {
    d: string;
};
export declare const e: {
    get f(): string;
    set f(val: number);
};

This is behaviour is not possible to reproduce for isolated declarations emitters, because it requires type comparisons for the types of the setter and getter.

🙂 Expected behavior

One of:

Emit based on syntax, do not collapse a getter/setter pair with the same type to a property

export declare const a: {
    get b(): string;
    set b(val: string);
};
export declare const c: {
    get d(): StringAlias;
    set d(val: string);
};
export declare const e: {
    get f(): string;
    set f(val: number);
};

OR

Error on both c.d and e.f because return type of getter and setter do not match

Additional information about the issue

cc @dragomirtitian

[Wesley Wigham]

Hm. We didn't allow get/set in types for a looong time, and only added it to support differing getter/setter type pairs - and to avoid accidentally breaking backcompat for people, we only emitted getter/setter pairs if the types of the getter and setter actually differed.

@RyanCavanaugh do you think we've supported getter-setter pairs in .d.ts files for long enough that we can just swap our emit to always retaining the getters/setters, rather than doing the property coalescing it does now?

[Jake Bailey]

I feel like we just talked about this in a recent design meeting or something; IMO we should be preserving things as-written and not be doing our merging thing anymore.

[Titian Cernicova-Dragomir]

I am actually surprised this does error. I remember adding an error if there is a type on both getter and setter, and only collapsing them if the type was specified only on one of them.

[Luca Casonato]

Just found another problem related to the current emit:

export const x = {
    set foo(str: string) {}
}

emits as:

export declare const x: {
    foo: string;
};

even though it should probably be

export declare const x: {
    set foo(str: string);
};

or

export declare const x: {
    get foo(): void;
    set foo(str: string);
};

[Jake Bailey]

Yeah, TS internally has no notion of a "set only property", see #58112 and #58119

(Void is definitely not the right answer, though)