Object.assign does not conclude correct type from second argumet in ternary operator

[aghArdeshir]

TypeScript Version: 4.0.0-dev

Search Terms: Object assign, spread

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
const production: boolean = true;

interface Icon {
    x: number;
    y: number;
    name: 'we' | 'you' | 'hello' | undefined;
}

const myBeautifulIcon: Icon = Object.assign(
    { x: 2, y: 2 },
    production ? { name: 'we' } : { name: 'you' }
)

console.log(myBeautifulIcon);

(code tested in nightly ts playground)

Expected behavior:
The code to be valid and myBeautifulIcon object have correct type. The correct type should be concluded from Object assign.

Actual behavior:
Error arised, saying that theconcluded type for name was string;

Playground Link:
https://www.typescriptlang.org/play/?ts=4.0.0-dev.20200609&ssl=1&ssc=1&pln=14&pc=30#code/MYewdgzgLgBADgJxAEwK7CgS3ALhgIxBABsBTAQzBgF4YoFVSBuAKBczClIQDNzhSMAJKgqAbxYwpMAB54wqALb5uraTACe8pSoRrpYcotJ4A5AHdSpmAB8YpjSFTW7pgBalixEC5iowyKQ8HKTIrAC+bKLQMIoaAEIUqFg8qMQiuMKiNDAA8vgAVqQYAHTkEBCYAOZgABSS0mKyeABMADSarTDhbQ1SiCjoWOAwAPwwTYbGZpbW4TB4k0Ym9o7O3SwAlFHgECSkJd5VtXGJ5MmYqemim0xAA

Related Issues:
#35621
#26792
#16163

More practical use case:

[aghArdeshir]

Just to make it slience:
https://www.typescriptlang.org/play/?ts=4.0.0-dev.20200609#code/MYewdgzgLgBADgJxAEwK7CgS3ALhgIxBABsBTAQzBgF4YoFVSBuAKBczClIQDNzhSMAJKgqAbxYwpMAB54wqALb5uraTACe8pSoRrpYcotJ4A5AHdSpmAB8YpjSFTW7pgBalixEC5iowyKQ8HKTIrAC+bKLQMIoaAEIUqFg8qMQiuMKiNDAA8vgAVqQYAHTkEBCYAOZgABSS0mKyeABMADSarTDhbQ1SiCjoWOAwAPwwTYbGZpbW5faz3TB4k0Ym9o7OMPMOTtaRAJRR4BAkpCXeVbVxieTJmKnpogdMQA

[j-oliveras]

You can also use as const since version 3.4:

    production ? { name: 'we' } as const : { name: 'you' } as const
    // or
    production ? { name: 'we' as const } : { name: 'you' as const }

[RyanCavanaugh]

Object.assign is "just a function" so the normal widening rules apply - basically TS's assumption is that Object.assign will leave the basic type alone, but might modify the primitive within its family (e.g. maybe Object.assign({ x: -5, y: "hello"}) is { x: 5, y:" HELLO" }). as const is the correct thing to write to indicate that you don't believe that will happen.

[aghArdeshir]

Oo! That's a bummer.
Cause lots of times, typescript does things that "feels like" smart!
And that feeling in me made that expecttion that I wished typescript to conclude result type.