NonNullable not properly casting generic type when excluding nullish values

[flavianh]

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

[jcalz]

Feels like #33912

[fatcerberus]

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

[RyanCavanaugh]

See e.g. #22348

[typescript-bot]

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