Bug Report

TypeScript narrows type wrong when using instanceof for generic class with optional field

🔎 Search Terms

typescript instanceof template optional narrowing

🕗 Version & Regression Information

It seems the bug present in all versions: old, actual and nightly

⏯ Playground Link

https://www.typescriptlang.org/play?target=8&ts=4.8.0-dev.20220602#code/MYGwhgzhAECiA8sB80DeAoaXoFMD8AXHANzoC+66okMAgvAJIo4AeALjgHYAmMCTGbNABmASxwhuRTgFcAtgCMcAJ2gBeaAAZSFKuCjRaARkbN2XXodNpM2MRO5HC0WYpU7K1A7QBM11hw8dNaCduKSRtLySqoaRh7owjKcwGyiAPacIqYAFGBE9K4xSACUNkKiwtB50KKcEGxgKTjpVcZloUJYYAB09hGkQhQVVTV1DU3ALW0+HbZd0L39joPYFBRAA

💻 Code

class E<E> {
    e?: E;
}

class A<I> extends E<I>{
    field: number = 0;
}

class A1<I> extends A<I> {
    field1?: number;
}

class A2<I> extends A<I> {
    field1: number = 1;
}

function f<I>(a: A<number>) {
    if (a instanceof A1) {
        a.field1;
    }
    if (a instanceof A2) {
        a.field1;
    }
}

🙁 Actual behavior

Property 'field1' does not exists in type A in first if, variable a narrowed to A

🙂 Expected behavior

variable a narrowed to A1 in first if and property field1 exists