TypeScript incorrectly reports literal comparison error inside class method after state mutation

[mcube-12139]

Type: Bug

1. Open Visual Studio Code with TypeScript enabled.
2. Create a .ts file and insert the following code:

class Reproducer {
    n: number = 0;

    constructor(public enabled: boolean) { }

    increase() {
        if (this.enabled) {
            ++this.n;
        }
    }

    hello() {
        if (this.n === 0) {
            console.log("Hello");
            this.increase();
            if (this.n === 1) {
                console.log(" world");
            }
        }
    }
}

3. Hover over or inspect the condition if (this.n === 1) inside the hello() method.
4. Observe the TypeScript diagnostic error:

"This comparison appears to be unintentional because the types '0' and '1' have no overlap. ts(2367)"

5. Expected behavior:

• No error should be reported.
• this.n may be mutated via this.increase() before the second comparison, so this.n === 1 is a valid and reachable condition.

6. Actual behavior:

• TypeScript incorrectly infers this.n as the literal type 0 within the entire if (this.n === 0) block, ignoring the mutation from this.increase().
• This results in a false-positive error indicating that 0 and 1 have no overlap.

7. Additional notes:

• This appears to be a control flow analysis issue where TypeScript does not properly account for side effects of method calls that mutate class state.
• The issue may relate to overly aggressive narrowing of this.n within the conditional scope.

VS Code version: Code 1.121.0 (f6cfa2ea2403534de03f069bdf160d06451ed282, 2026-05-19T11:51:53+02:00)
OS version: Windows_NT x64 10.0.19045
Modes: Restricted

System Info

Item	Value
CPUs	12th Gen Intel(R) Core(TM) i5-12600KF (16 x 3686)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x10de, DEVICE=0x2803 [NVIDIA GeForce RTX 4060 Ti], DRIVER_VENDOR=NVIDIA, DRIVER_VERSION=32.0.15.6094 ACTIVE GPU1: VENDOR= 0x10de, DEVICE=0x2803 [NVIDIA GeForce RTX 4060 Ti], DRIVER_VERSION=32.0.15.6094 GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.5794 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	63.84GB (51.89GB free)
Process Argv	--crash-reporter-id 999d7587-7a41-4103-ac56-5e8d1d131ec7
Screen Reader	no
VM	0%

Extensions (1)

Extension	Author (truncated)	Version
vscode-icons	vsc	12.18.0

(1 theme extensions excluded)

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[RyanCavanaugh]

#9998