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Description

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

Contributor guide

Research direction: Investigate the type narrowing logic in the TypeScript checker when using instanceof on generic classes with optional properties. Focus on the flow control narrowing for class constructor signatures and generic type parameters.
Tech stack: typescript
Domain: tooling
Issue type: Bug
Difficulty: 3
Estimated time: Half day
Activity status: Active
Clarity: Clear
Prerequisites: TypeScriptTypeScript compiler internals
Newbie friendliness: 30