abstract type guard does not narrow when keyed with a unique symbol

Description

const isBar = Symbol("isBar");

abstract class Foo {
    abstract isBar(): this is Bar;
    abstract [isBar](): this is Bar;

    method(): void {
        if (this[isBar]()) {
            // This should not be an error, it should work the same as .isBar()
            this.barMethod();
        } else if (this.isBar()) {
            this.barMethod();
        }
    }
}

class Bar extends Foo {
    override isBar(): this is Bar {
        return true;
    }

    override [isBar](): this is Bar {
        return true;
    }

    barMethod(): void {}
}

Contributor guide

Tech stack

typescript

Domain

tooling

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-2 days

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

TypeScript type systemtype guardsabstract classesunique symbols

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

35

Research direction

Start by examining the TypeScript compiler's type narrowing logic in `src/compiler/checker.ts`, focusing on how computed property names with unique symbols are handled compared to regular method calls. The test case shows that calling `this.isBar()` works, but `this[isBar]()` does not, so look for how abstract type guards are resolved for symbol keys. Check if there is a missing case in `narrowTypeByMethodCall` or related functions. Also review existing tests for abstract type guards to understand expected behavior.