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

functiontest<Textendsstring|null>(input:T):NonNullable<T>{if(!input){// this should narrow `input` to `T extends string`thrownewError();}returninput;// 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 theNonNullable type.

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