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Generic cannot be narrowed to satisfy NonNullable Β #48691

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@longzheng

Description

@longzheng

Bug Report

πŸ”Ž Search Terms

  • generic can't be narrowed
  • narrow nonnullable generic

πŸ•— 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

πŸ’» 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
}

πŸ™ 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>'.

πŸ™‚ 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.

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