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Inference fails for generic-typed constructor parameter when no explicit constructor is present #47340

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#47750
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Inference fails for generic-typed constructor parameter when no explicit constructor is present#47340
#47750
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BugA bug in TypeScriptHelp WantedYou can do this
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Description

@nex3
Natalie Weizenbaum (nex3)
opened on Jan 7, 2022

Bug Report

I'm trying to write a function that takes a (generic) constructor and calls it to instantiate a class. The type inference for such a function normally works fine, but if the constructor is for a generic class it fails even if the generic parameter has a default.

🔎 Search Terms

generic constructor parameter inference instanceof

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about constructor parameters

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo<K extends {} = {}> {}

function create<T>(type: new () => T): T {
    return new type();
}

let string = create(String); // has type String

let foo = create(Foo); // should have type Foo<{}>, has type unknown

🙁 Actual behavior

The foo variable has type unknown.

🙂 Expected behavior

I expected the foo variable to have type Foo<{}>.

Activity

  1. cijiugechu commented on Jan 7, 2022

    @cijiugechu
    Uni Hikousen (cijiugechu)
    on Jan 7, 2022

    Probably because the class Foo don't have a constructor, K didn't be consumed in the class.

  2. MartinJohns commented on Jan 7, 2022

    @MartinJohns
    Martin Johns (MartinJohns)
    on Jan 7, 2022
    Contributor

    Use this instead:

    function create<T extends new () => any>(type: T): InstanceType<T> {
    return new type();
}

    Also, be wary of the String type. It's not assignable to string.

  3. Natalie Weizenbaum (nex3)
    added 2 commits that reference this issue on Jan 10, 2022

    Use InstanceType to avoid microsoft/TypeScript#47340

    9e04e0f

    Use InstanceType to avoid microsoft/TypeScript#47340 (#1765)

    c23369d
  4. Ryan Cavanaugh (RyanCavanaugh)
    added
    BugA bug in TypeScript
    on Jan 12, 2022

  5. RyanCavanaugh commented on Jan 12, 2022

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Jan 12, 2022
    Member

    This is a bug; inserting the constructor that's "there" by default

    constructor() {}

    changes the behavior

  6. Ryan Cavanaugh (RyanCavanaugh)
    added this to the Backlog milestone on Jan 12, 2022
  7. Ryan Cavanaugh (RyanCavanaugh)
    added
    Help WantedYou can do this
    on Jan 12, 2022
  8. Ryan Cavanaugh (RyanCavanaugh)
    changed the title [-]Inference fails for generic-typed constructor parameter[/-] [+]Inference fails for generic-typed constructor parameter when no explicit constructor is present[/+] on Jan 12, 2022
  9. Wesley Wigham (weswigham)
    closed this as completedin #47750on Feb 15, 2022
  10. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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