Generic cannot be narrowed to satisfy NonNullable

[longzheng]

Bug Report

🔎 Search Terms

• generic can't be narrowed
• narrow nonnullable generic

🕗 Version & Regression Information

• This is the behavior in every version I tried from 4.6.2 to 4.0.5 and nightly v4.7.0-dev.20220408

⏯ Playground Link

Playground link with relevant code

💻 Code

function test<T extends string | null>(input: T): NonNullable<T> {
  if (!input) { // this should narrow `input` to `T extends string`
    throw new Error();
  }

  return input; // type error
}

🙁 Actual behavior

Type error

Type 'T' is not assignable to type 'NonNullable<T>'.
  Type 'string | null' is not assignable to type 'NonNullable<T>'.
    Type 'null' is not assignable to type 'NonNullable<T>'.

🙂 Expected behavior

TypeScript should know I've narrowed the generic type to satisfied the NonNullable type.

I came across #43183 which I was expecting to also address this scenario.

[longzheng]

This might be a duplicate of #48048

[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.