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Optional discriminated union's callback parameter type is not inferred #46847

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Optional discriminated union's callback parameter type is not inferred#46847
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BugA bug in TypeScriptDomain: check: Type InferenceRelated to type inference performed during signature resolution or `infer` type resolution
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Backlog

Description

@wzono
Wesley Zon (wzono)
opened on Nov 18, 2021

Bug Report

Optional discriminated union's callback parameter type is not inferred

🔎 Search Terms

#35769

🕗 Version & Regression Information

[email protected]

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.5.0-beta#code/C4TwDgpgBAglC8UDeAoK6qkgfgFxQHIYCAaNDAQ3wAoBjfJKAE3wDsBXAWwCMIAnKAF8AlAgB8UAG4B7AJZMA3CkEoUWaACEEycunX4CG0rqhUodBszZdefElHpQAzsD6zWAcyGj4EmfKUVVXVYDQBhbTgAHygtJRRJCgEzGHCAbQBdKBiAJQhaaT4mAB4XN097VLDMiUQ0k1QMJtMaMCSKTicfCUbmvpMmlUGUDKA

💻 Code

type A = {
    type?: 'A',
    a: (c: { d: number }) => void;
}

type B = {
    type: 'B',
    a: (c: { d: number, c: string }) => void;
}


type ABC = A | B ;

var a: ABC[] | Record<string, ABC[]> = [
    {
        a: (params) => {
            
        }
    }
]

🙁 Actual behavior

Parameters of method 'a' can not be inferred correctly.

🙂 Expected behavior

Parameters of method 'a 'can be inferred correctly: {d: number}

Activity

  1. Ryan Cavanaugh (RyanCavanaugh)
    added
    BugA bug in TypeScript
    on Nov 18, 2021
  2. Ryan Cavanaugh (RyanCavanaugh)
    added this to the Backlog milestone on Nov 18, 2021

  3. RyanCavanaugh commented on Nov 18, 2021

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Nov 18, 2021
    Member

    Behavior dates back to at least 3.3.3. It looks like our discriminant detection is getting a little confused by the Record part of the union.

  4. wzono commented on Dec 10, 2021

    @wzono
    Wesley Zon (wzono)
    on Dec 10, 2021
    Author

    Ryan Cavanaugh (@RyanCavanaugh) Any progress?

  5. RyanCavanaugh commented on Dec 13, 2021

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Dec 13, 2021
    Member

    @wingsico no

  6. Eldemarkki commented on Aug 4, 2023

    @Eldemarkki
    Eetu Mäenpää (Eldemarkki)
    on Aug 4, 2023

    Any updates?

    I'm not sure if this Playground link is the same problem as this, but if true, then still exists in 5.1.6: https://www.typescriptlang.org/play?ts=5.1.6#code/C4TwDgpgBAEghgOwCYBsICcAKd1wLYTAYDOUAvFAN4BQUdUAjgK4YgBqcKAlknMAPboAXFAAUASnIA+KACN+-NIgDctegAtEqDCNHNWIylB4jiwdFwQBzKAF9JZGQDd+PVbagAfKmrr707Jw8fIIA-CJMyBAAZpYQSKr0UJrIaMJi-iARUbEI8Q7OrgnUtqrUAMb8CGZQYDj4pBQ0SZkc3LwC6RLSUOYsADS+yVppupkFPklJldWKEAB0KPxWeiwB4kO2tB7EfFzEsRCk8KkY2LgEROjEqkA

  7. Ryan Cavanaugh (RyanCavanaugh)
    added
    Domain: check: Type InferenceRelated to type inference performed during signature resolution or `infer` type resolution
    on Oct 16, 2025
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