Strange behavior of function overloading and union type

[Jack-Works]

Bug Report

Strange behavior of function overloading and union type

🔎 Search Terms

function overloading union type

🕗 Version & Regression Information

• This changed between versions 3.5 (good) and 3.6 (bad)

⏯ Playground Link

Playground link with relevant code

💻 Code

import { Canvas, CanvasRenderingContext2D as NodeCanvasRenderingContext2D } from 'canvas'

let ctx: CanvasRenderingContext2D | NodeCanvasRenderingContext2D | null = null

function test(canvas: HTMLCanvasElement | Canvas) {
    ctx = canvas.getContext('2d')
    // error??
}
function test2(canvas: HTMLCanvasElement | Canvas) {
    // works!!
    if (canvas instanceof HTMLCanvasElement) {
        ctx = canvas.getContext('2d')
    } else {
        ctx = canvas.getContext('2d')
    }
}

🙁 Actual behavior

Type 'NodeCanvasRenderingContext2D | RenderingContext | null' is not assignable to type 'CanvasRenderingContext2D | NodeCanvasRenderingContext2D | null'.

🙂 Expected behavior

No error

[RyanCavanaugh]

Function type merging produces a single signature in this case, and that single signature doesn't keep around enough information.

The only way to process this "correctly" would be combinatorially explosive, so that doesn't happen.

[Jack-Works]

but why does it work on TS 3.5? 🤔