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Symbol properties should not be exempt from excess property checks in presence of a string index signature #44794

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DanielRosenwasser opened this issue Jun 28, 2021 · 4 comments · May be fixed by #58190
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Symbol properties should not be exempt from excess property checks in presence of a string index signature #44794

DanielRosenwasser opened this issue Jun 28, 2021 · 4 comments · May be fixed by #58190
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@ahejlsberg
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Breaking Change Would introduce errors in existing code Bug A bug in TypeScript Fix Available A PR has been opened for this issue Rescheduled This issue was previously scheduled to an earlier milestone
Milestone
TypeScript 5.5.0

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@DanielRosenwasser
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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.
@DanielRosenwasser DanielRosenwasser mentioned this issue Jun 28, 2021
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@andrewbranch
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Should symbols be exempt from excess property checks in presence of a string index signature?

Nope

@DanielRosenwasser
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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
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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 DanielRosenwasser added Breaking Change Would introduce errors in existing code Bug A bug in TypeScript labels Jun 28, 2021
@DanielRosenwasser DanielRosenwasser added this to the TypeScript 4.5.0 milestone Jun 28, 2021
@DanielRosenwasser
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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.

@DanielRosenwasser DanielRosenwasser changed the title Should symbols be exempt from excess property checks in presence of a string index signature? Symbol properties should not be exempt from excess property checks in presence of a string index signature Jun 28, 2021
@liuweiGL liuweiGL mentioned this issue Jun 29, 2021
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@ahejlsberg ahejlsberg mentioned this issue Jun 29, 2021
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@RyanCavanaugh RyanCavanaugh added the Rescheduled This issue was previously scheduled to an earlier milestone label Dec 15, 2021
@MartinJohns MartinJohns mentioned this issue Mar 15, 2023
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@jcalz jcalz mentioned this issue Jul 17, 2023
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@Emiyaaaaa Emiyaaaaa mentioned this issue Sep 6, 2023
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@Andarist Andarist linked a pull request Apr 14, 2024 that will close this issue
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@typescript-bot typescript-bot added the Fix Available A PR has been opened for this issue label Apr 14, 2024
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@ahejlsberg ahejlsberg

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Breaking Change Would introduce errors in existing code Bug A bug in TypeScript Fix Available A PR has been opened for this issue Rescheduled This issue was previously scheduled to an earlier milestone
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Don't exempt late bound symbols from EPC in presence of string index signature Andarist/TypeScript
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@DanielRosenwasser @andrewbranch @ahejlsberg @RyanCavanaugh @typescript-bot