Inconsistent errors reported between tsc and VSCode (Unions across objects with method overload definitions)

Bug Report

This is similar to #52813, in spirit. However the errors, how it's reproduced, and the code causing it are pretty different.

Basically I have a code snippet that appears 100% error-free* (most of the time, see note below) in the VSCode IDE. Logically in my head the code should be error free too. However, when I run tsc I get an error inside the snippet.

Here's the code:

export function createWebGLContext(
  canvas: HTMLCanvasElement | OffscreenCanvas,
): WebGLRenderingContext | null {
  return (
    canvas.getContext('webgl')
  )
}

tsc reports this error:

createWebGLContext.ts:4:3 - error TS2322: Type 'RenderingContext | null' is not assignable to type 'WebGLRenderingContext | null'.
  Type 'CanvasRenderingContext2D' is missing the following properties from type 'WebGLRenderingContext': drawingBufferHeight, drawingBufferWidth, activeTexture, attachShader, and 428 more.

4   return (
    ~~~~~~~~
5     canvas.getContext('webgl')
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
6   )
  ~~~

* In the larger project that I originally discovered this issue, I actually do sometimes see the error in the IDE. I haven't figured out how to consistently cause it, but restarting the TS server or making a small change and saving the file generally clears the error.

🔎 Search Terms

inconsistency, inconsistent, errors, ts language server, tsc

🕗 Version & Regression Information

• Attempted on 5.0.3 and latest dev build (typescript@5.1.0-dev.20230331)

⏯ Playground Link

With the Playground, since it uses elements of VSCode under the hood, the snippet appears valid with no errors.
Playground link with relevant code

You can also clone this repo or start a Github CodeSpace on it to reproduce the errors when running tsc:
https://github.com/frank-weindel/ts-53614-ide-tsc-inconsistency

🙁 Actual behavior

• tsc considers the above code to have an error.
• VSCode normally and properly does not show that error, but it occasionally can.

🙂 Expected behavior

• tsc should not consider above code to have an error.
• VSCode should be consistent with tsc.

Analysis

I'm not sure why but this seems related to the use of the union HTMLCanvasElement | OffscreenCanvas for the variable canvas.

Both interfaces in the union contain a set of method overloads for getContext...

lib.dom.d.ts

// ...
interface HTMLCanvasElement extends HTMLElement {
    // ...
    getContext(contextId: "2d", options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D | null;
    getContext(contextId: "bitmaprenderer", options?: ImageBitmapRenderingContextSettings): ImageBitmapRenderingContext | null;
    getContext(contextId: "webgl", options?: WebGLContextAttributes): WebGLRenderingContext | null;
    getContext(contextId: "webgl2", options?: WebGLContextAttributes): WebGL2RenderingContext | null;
    getContext(contextId: string, options?: any): RenderingContext | null;
    // ...
}
// ...
interface OffscreenCanvas extends EventTarget {
    // ...
    getContext(contextId: "2d", options?: any): OffscreenCanvasRenderingContext2D | null;
    getContext(contextId: "bitmaprenderer", options?: any): ImageBitmapRenderingContext | null;
    getContext(contextId: "webgl", options?: any): WebGLRenderingContext | null;
    getContext(contextId: "webgl2", options?: any): WebGL2RenderingContext | null;
    getContext(contextId: OffscreenRenderingContextId, options?: any): OffscreenRenderingContext | null;
    // ...
}
// ...

Based on these method overload signatures I'd expect the statement canvas.getContext('webgl') to use the union of these two overrloads:

getContext(contextId: "webgl", options?: any): WebGLRenderingContext | null; // from HTMLCanvasElement
getContext(contextId: "webgl", options?: WebGLContextAttributes): WebGLRenderingContext | null; // from OffscreenCanvas

Which I believe would reduce down to look like this:

getContext(contextId: "webgl", options?: any): WebGLRenderingContext | null;

Where the return type is WebGLRenderingContext | null. Which is what we see if we hover over the method name in the Playground or VSCode.

However, tsc inferred the return type of the method to be RenderingContext | null.

RenderingContext | null is returned by the base method signature for getContext() in HTMLCanvasElement:

getContext(contextId: string, options?: any): RenderingContext | null;

So it would seem tsc somehow is inferring the wrong override while TS Language Server / VSCode is inferring the correct one.

Start with the linked reproduction repository and compare the tsc result with the VSCode or Playground result for the HTMLCanvasElement | OffscreenCanvas example. Inspect the lib.dom.d.ts overloads and the compiler path that resolves the union call; done means tsc and the language service agree, with a regression test covering the reported snippet.