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NonNullable not properly casting generic type when excluding nullish values #48048

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#49119
Closed
NonNullable not properly casting generic type when excluding nullish values#48048
#49119
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DuplicateAn existing issue was already createdFix AvailableA PR has been opened for this issue

Description

@flavianh
Flavian Hautbois (flavianh)
opened on Feb 26, 2022

Bug Report

🔎 Search Terms

Generic types nonnullable

🕗 Version & Regression Information

This bug appears to exist ever since NonNullable was introduced (tested from 3.3.3 to 4.5.4)

⏯ Playground Link

Playground link with relevant code

💻 Code

function throwIfNullable<T>(
  value: T,
): NonNullable<T> {
  if (value !== undefined && value !== null) {
    return value;
  }

  throw Error('Nullable')
}

🙁 Actual behavior

The line return value raises a compilation error Type 'T' is not assignable to type 'NonNullable<T>'.

🙂 Expected behavior

No compilation error as the if clause properly guards against nullish values

Activity

  1. Flavian Hautbois (flavianh)
    changed the title [-]NonNullable not properly casting generic types when excluding nullish values[/-] [+]NonNullable not properly casting generic type when excluding nullish values[/+] on Feb 26, 2022

  2. jcalz commented on Feb 26, 2022

    @jcalz
    Joe Calzaretta (jcalz)
    on Feb 26, 2022
    Contributor

    Feels like #33912

  3. fatcerberus commented on Feb 26, 2022

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Feb 26, 2022

    An alternative construction that works would be:

    function throwIfNullish<T>(value: T | null | undefined): asserts value is T
{
    if (value == null) throw Error("Nullish!");
}

declare let x: string | undefined;
throwIfNullish(x);  // x: string | undefined
x;  // x: string

  4. RyanCavanaugh commented on Feb 28, 2022

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Feb 28, 2022
    Member

    See e.g. #22348

  5. Ryan Cavanaugh (RyanCavanaugh)
    added
    DuplicateAn existing issue was already created
    on Feb 28, 2022
  6. TypeScript Bot (typescript-bot)
    closed this as completedon Mar 3, 2022

  7. typescript-bot commented on Mar 3, 2022

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Mar 3, 2022
    Contributor

    This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

  8. TypeScript Bot (typescript-bot)
    added
    Fix AvailableA PR has been opened for this issue
    on May 20, 2022
  9. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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