Union type to copy properties to other object not assignable to never

[greg-hornby-roam]

TypeScript Version: 3.5.x

Search Terms: union properties object never

Code

interface IUser {
    id: number;
    createdAt: Date;
    name: string;
    age: number;
    isActive: boolean;
}

//Imagine this array is an array of strings matching the property names of IUser
let properties = [] as any[] as (keyof IUser)[];

let user: IUser = {} as any; //Imagine this being fetched from a database
let constructedUser: IUser = {} as any;
for (let prop of properties) {
    constructedUser[prop] = user[prop];
}

Expected behavior: No errors

Actual behavior:

Type 'string | number | boolean | Date' is not assignable to type 'never'.
  Type 'string' is not assignable to type 'never'.

This used to work on 3.4, seems broken on 3.5

Playground Link:
3.5: (Broken)
https://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgJIFUDO1kG8BQyRywAJgFzIgCuAtgEbQDchxCUEckpAgmJQBEuEFsSpxaESpjBRQAc1HE48qVTqMoSosEw8EYYADc19APZmANpxAsAvvnwB6J6lorQKMAAtdyOFBQcACeJJj+IP6BIchmMMgyciDy4e5gCL7JyD4oAA5QZrnQYKEgEhDhcWhY0PjWYMj5hcXAFcgAvMgA2gC6-uFwIMG9-cgAFADWEMFVGNhQAJS9LHUQDdTzlHM4nbh2o4PBTMgubh4gXr7hjArIMGsZEKR3BbT+yKRccPRw2KsNCDMIES1AMTxqUC2EI6eH2vwiR3wMDMUHG9UaBVysXiTSKUEMFQWeFYREBwNkoO4EK6uL6nQ20BpmJ69iAA

3.3: (Working)
https://www.typescriptlang.org/play/?ts=3.3.3#code/JYOwLgpgTgZghgYwgAgJIFUDO1kG8BQyRywAJgFzIgCuAtgEbQDchxCUEckpAgmJQBEuEFsSpxaESpjBRQAc1HE48qVTqMoSosEw8EYYADc19APZmANpxAsAvvnwB6J6lorQKMAAtdyOFBQcACeJJj+IP6BIchmMMgyciDy4e5gCL7JyD4oAA5QZrnQYKEgEhDhcWhY0PjWYMj5hcXAFcgAvMgA2gC6-uFwIMG9-cgAFADWEMFVGNhQAJS9LHUQDdTzlHM4nbh2o4PBTMgubh4gXr7hjArIMGsZEKR3BbT+yKRccPRw2KsNCDMIES1AMTxqUC2EI6eH2vwiR3wMDMUHG9UaBVysXiTSKUEMFQWeFYREBwNkoO4EK6uL6nQ20BpmJ69iAA

Related Issues:

[fatcerberus]

This is a consequence of #30769.

[greg-hornby-roam]

I guess it's a design limitation that if the target and source objects are the same interface, and are being indexed with the same key it can't infer that in that one case the types should match?

[fatcerberus]

The issue is that the compiler only sees the types when checking this - it doesn’t know that prop evaluates to the same key on both sides, only that it’s type keyof IUser. That isn’t enough information to guarantee the assignment is safe, so it rejects it.

See also: #31445

[typescript-bot]

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