Generic type-safe implementation for Object.assign

[Schniz]

It seems that it can be possible to make a type-safe Object.assign-like functions with TypeScript, without having to make lots of overrides.

Here's an example on the playground:

https://www.typescriptlang.org/play/index.html#src=function%20assign%3CSource%20extends%20%7B%7D%2C%20Result%20extends%20Source%3E(%0D%0A%20%20%20%20target%3A%20Source%2C%0D%0A%20%20%20%20...sources%3A%20Source%5B%5D%0D%0A)%3A%20Pick%3CResult%2C%20keyof%20Source%3E%20%7B%0D%0A%20%20%20%20return%20Object.assign(target%2C%20...sources)%3B%0D%0A%7D%0D%0A%0D%0Alet%20x%20%3D%0D%0A%20%20%20%20assign(%0D%0A%20%20%20%20%20%20%20%20%7B%20hello%3A%20%22world%22%20%7D%2C%0D%0A%20%20%20%20%20%20%20%20%7B%20something%3A%20%22is%20good%22%20%7D%2C%0D%0A%20%20%20%20%20%20%20%20%7B%20%22yes%22%3A%20true%20%7D%2C%0D%0A%20%20%20%20%20%20%20%20%7B%20yes%3A%20%22hello%22%20%7D%0D%0A%20%20%20%20)%3B%0D%0A%0D%0Ax.hello%3B%0D%0Ax.world%3B%20%2F%2F%20error!%0D%0Ax.yes%3B

You can see that the compiler says x.hello is fine, and complains about x.world

[typescript-bot]

It looks like you've sent a pull request to update our 'lib' files. These files aren't meant to be edited by hand, as they consist of last-known good states of the compiler and are generated from 'src'. Unless this is necessary, consider closing the pull request and sending a separate PR to update 'src'.

[msftclas]

All CLA requirements met.

[typescript-bot]

It looks like you've sent a pull request to update our 'lib' files. These files aren't meant to be edited by hand, as they consist of last-known good states of the compiler and are generated from 'src'. Unless this is necessary, consider closing the pull request and sending a separate PR to update 'src'.

[typescript-bot]

It looks like you've sent a pull request to update our 'lib' files. These files aren't meant to be edited by hand, as they consist of last-known good states of the compiler and are generated from 'src'. Unless this is necessary, consider closing the pull request and sending a separate PR to update 'src'.

[j-oliveras]

Your change don't work correctly (playground):

function assign<Source extends {}, Result extends Source>(
    target: Source,
    ...sources: Source[]
): Pick<Result, keyof Source> {
    return Object.assign(target, ...sources);
}

let y1 = assign(
    () => "",
    {
        version: 1
    }
);

let y2 = Object.assign(
    () => "",
    {
        version: 1
    }
);

y1 (using your change) has an error on version property:

Argument of type '{ version: number; }' is not assignable to parameter of type '() => string'.
  Object literal may only specify known properties, and 'version' does not exist in type '() => string'.

but y2 (using existing Object.assign definition) has no problems.

[RyanCavanaugh]

It'd be best to start with an issue describing non-working scenarios and outlining tests. We can't really evaluate a PR without first understanding why it's being made. I think the comment from @j-oliveras is instructive - this would likely be a large compat problem.