Incorrect type inference when using spread operator with index signature and adding another prop

[danmana]

Bug Report

🔎 Search Terms

typescript spread wrong type inference

🕗 Version & Regression Information

• This is the behavior in every version I tried (ex: 3.3.3-4.7.4)

⏯ Playground Link

Playground link with relevant code

💻 Code

interface A {
    title: string
    [year: number]: number
}
let a: A = {title: 'A', 2020: 123};

let b = { ...a}
let c = { ...a, label: 'revenue'}

🙁 Actual behavior

// d.ts inference
declare let b: {
    [x: number]: number;
    title: string;
};
declare let c: {
    label: string;
    title: string;
};

🙂 Expected behavior

// d.ts inference
declare let b: {
    [x: number]: number;
    title: string;
};
declare let c: {
    [x: number]: number; // <--- this is missing
    label: string;
    title: string;
};

It seems that when I add an extra property to the spread object, it loses the index signature properties from the spread source.
But, if I do not add any extra property it does infer correctly the type...

[MartinJohns]

Duplicate of #27273. Used search terms: spread index in:title

[danmana]

Yes, I think you are right and it's a duplicate ... sorry I missed that

For context, this situation is slightly different as there is no overlap in the keys [x: number]: number and label: string, unlike the linked issue which uses Record<string, string> and a: '' where there is no obvious resolution.
I'm not sure if this is a relevant enough distinction to keep this issue open.

For reference, as mentioned in the comments there, Object.assign does produce an expected result

let d = Object.assign({}, a, {label: 'revenue'})

// infered d.ts
declare let d: A & {
    label: string;
};

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.