Aliases are lost when conditionals are applied

Mateusz Burzyński (@Andarist)

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aliases lost

🕗 Version & Regression Information

Discussion with Mateusz Burzyński (@Andarist):

TypeFormatFlags.InTypeAlias is at fault here, it's rather hard to track its exact conceptual purpose. And it has been there forever - I found commits with it from 9 years ago
I briefly thought this might be related to ur issue: Type alias printing regressed in TypeScript 4.2.2 #43031
but now I'm pretty sure it isn't
but it's also a somewhat funny issue because it complains about an alias for a union being lost and it was broken by a PR that... introduces improved union/intersection aliasing 😄 Preserve type aliases for union and intersection types #42149 , there was also a quick follow up to this PR that feels slightly related: Support re-aliasing of type alias instantiations #42284

Anyway, I'm sort of leaving breadcrumbs for myself - sorry for oversharing 😉 I'll dig further later

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAShDOBXANsAPAQQDRQKID4oBeKAbwCgoqoAnCAQwBMB7AO2RCgH1h6BzAFxQARAHkA1sMrU6TNhygA3eskQQhGcgF8oAHzLSqslu049+Q4QDF6AS2RTqtBiYUQaNZjSG5t5cowQAMbI9HRQQWzwwFC28HBIqEIAFIhCiKzirMwA7qwAlMSEiLHwsAgo6PSsIDjVIPj+kazRUABmrMRQmDgEybasYEIJlT14hPrRNAN8hUSEcSOo-YOFAPyxg1CWzJJAA

💻 Code

type Result<A, E> = {
    readonly _tag: "Ok"
    readonly value: A
} | {
    readonly _tag: "Fail"
    readonly error: E
}

declare const isResult: (u: unknown) => u is Result<any, any>

const fn = <A, E>(inp: Result<A, E> | string) => isResult(inp) ? inp : "ok"

🙁 Actual behavior

const fn: <A, E>(inp: Result<A, E> | string) => {
    readonly _tag: "Ok";
    readonly value: A;
} | {
    readonly _tag: "Fail";
    readonly error: E;
} | "ok"

🙂 Expected behavior

const fn: <A, E>(inp: Result<A, E> | string) => Result<A, E> | "ok"

Additional information about the issue

No response

Mateusz Burzyński (@Andarist)

Correction, breadcrumbs from Mateusz Burzyński (@Andarist) appears to be unrelated, they instead relate to the following code where it is only the printer at fault, instead in this case aliases are completely lost:

import { Effect, Unify, Option } from "effect"

type Working = Unify.Unify<Effect.Effect<number> | Effect.Effect<string>>
//   ^? - Effect.Effect<number | string, never, never>

type NotWorking = Unify.Unify<Option.Option<number> | Option.Option<string>>
//   ^? - Option.None<number | string> | Option.Some<number | string>

Fixing this requires making getNarrowedTypeWorker smarter about .origin preservation. It gets lost in this, somewhat unusual, mapType call here:
https://github.dev/microsoft/TypeScript/blob/4dc677b292354f4b9162452b2e00f4d7dd118221/src/compiler/checker.ts#L29346-L29365

Maybe some special path has to be introduced to handle this but maybe it's possible to restructure this somehow so the existing code paths responsible for .origin preservation could kick in.

Closing language service bugs related to the 6.0 implementation. For more information, see #62827

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Aliases are lost when conditionals are applied #61581

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Aliases are lost when conditionals are applied #61581

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🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

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mikearnaldi commented on Apr 15, 2025

Andarist commented on Apr 15, 2025

RyanCavanaugh commented on Dec 3, 2025

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