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5.0: const-like inference is not preserved unless a mapped type is usedΒ #53307

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#53341
Closed
5.0: const-like inference is not preserved unless a mapped type is used#53307
#53341
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ahejlsberg
Labels
BugA bug in TypeScriptFix AvailableA PR has been opened for this issue
Milestone
TypeScript 5.1.0

Description

@conorbrandon
Conor Brandon (conorbrandon)
opened on Mar 17, 2023

Bug Report

πŸ”Ž Search Terms

const type, validator type

πŸ•— Version & Regression Information

5.0.2

  • I was unable to test this on prior versions because const type parameters are a new feature

⏯ Playground Link

Playground link with relevant code

πŸ’» Code

type NotEmpty<T extends Record<string, any>> = keyof T extends never ? never : T;
const thing = <const O extends Record<string, any>>(o: NotEmpty<O>) => {
  return o;
};
const t = thing({foo: ''});
// is inferred as { foo: string } ❌

// BUT, when adding a mapped type to NotEmpty it works
type NotEmptyMapped<T extends Record<string, any>> = keyof T extends never ? never : {
  [K in keyof T]: T[K] // ⬅️
};
const thingMapped = <const O extends Record<string, any>>(o: NotEmptyMapped<O>) => {
  return o;
};
const tMapped = thingMapped({foo: ''});
// is inferred as { foo: "" } βœ…

πŸ™ Actual behavior

When using a non-mapped validator type, the "valid" branch does not preserve the const-like inference unless the type is mapped over.

πŸ™‚ Expected behavior

A mapped type is not needed to preserve const-like inference.

Activity

  1. Andarist commented on Mar 17, 2023

    @Andarist
    Mateusz BurzyΕ„ski (Andarist)
    on Mar 17, 2023
    Contributor

    Working case:

    • the apparent type of contextual type for this object is { [K in keyof O]: O[K]; }
    • thus the type for this property becomes O["foo"]
    • isConstTypeParameterContext is able to return true based on this information

    Not working case:

    • the apparent type of contextual type for this object is Record<string, any>
    • thus the type for this property becomes any
    • isConstTypeParameterContext return false based on that

  2. conorbrandon commented on Mar 17, 2023

    @conorbrandon
    Conor Brandon (conorbrandon)
    on Mar 17, 2023
    Author

    Interesting! Thanks for taking a look.

    I did find it a bit curious that NotEmptyMapped unfortunately doesn't seem to actually error for an empty object, and instead returns Record<string, any>. Is that possibly related?

    Both correctly error if the check is instead changed to {} extends T, but have the same const-like inference mismatch.

    Finally if the check is T extends Record<string, never>, both don't use const-like inference and NotEmptyMapped doesn't error on an empty object, but NotEmpty does.

    I think that's all the ways I can think of to check for an empty object...

  3. Anders Hejlsberg (ahejlsberg)
    self-assigned this
    on Mar 17, 2023
  4. Anders Hejlsberg (ahejlsberg)
    added
    BugA bug in TypeScript
    on Mar 17, 2023

  5. ahejlsberg commented on Mar 17, 2023

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Mar 17, 2023
    Member

    It looks like we need to probe types a little deeper in isConstTypeVariable.

  6. Anders Hejlsberg (ahejlsberg)
    added this to the TypeScript 5.1.0 milestone on Mar 17, 2023
  7. TypeScript Bot (typescript-bot)
    added
    Fix AvailableA PR has been opened for this issue
    on Mar 18, 2023
  8. Anders Hejlsberg (ahejlsberg)
    closed this as completedin #53341on Apr 1, 2023
  9. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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