Symbol properties should not be exempt from excess property checks in presence of a string index signature

[DanielRosenwasser]

In #44793, we found that a symbol index signature doesn't turn off excess property checks on symbols; however, a string index signature turns these excess property checks entirely.

interface A {
    a?: string
    b?: number
    [key: string]: unknown
}

const symbolKey = Symbol('key')

const a: A = {
    [symbolKey]: '123', // works!
}

It's likely that some users relied on this behavior; however, those users will be able to use symbol index signatures in TypeScript 4.4.

[andrewbranch]

Should symbols be exempt from excess property checks in presence of a string index signature?

Nope

[DanielRosenwasser]

Well

It's likely that some users relied on this behavior

and from @ahejlsberg's comment at #44793 (comment)

It's an easy fix, but the odd thing is that it ends up being a breaking change because we previously would validate symbol-named properties again string index signatures when performing excess property checking.

[andrewbranch]

Well I assumed you’d weigh my “nope” against other opinions and data, but it seems like a good breaking change to me (one I’d vet in 4.5, unless we want to try to backport it into 4.4-beta).

[DanielRosenwasser]

I would prefer not picking that into 4.4 at this point. My preference is to give people a step-wise upgrade pattern. Ship one version with the string indexer behavior, and then rip off the band-aid in 4.5.