Seems like a relatively minor change to the JS API, may be worth bringing up at a future CG meeting.

Currently, WebAssembly.Module, WebAssembly.Instance and others break this expectation by always returning instance of the built-in class.

Not from the constructors, they don't. See the steps in https://heycam.github.io/webidl/#create-an-interface-object and https://heycam.github.io/webidl/#internally-create-a-new-object-implementing-the-interface in particular. I wouldn't be surprised if there were implementation bugs, of course. I'll make a note to add tests at some point.

Open question: what to do with static APIs like WebAssembly.compile{Streaming} / WebAssembly.instantiate{Streaming}?

Unfortunately, these don't live on corresponding classes, so they can't guess which target class user wanted.

There are few options:

1. Add their duplicates to the corresponding classes as `WebAssembly.Module.compile{Streaming}` / `WebAssembly.Instance.instantiate{Streaming}`, so that they would always have access to specific context. Not pretty, but would integrate more naturally with subclassing.

2. Pass target class(es) in an extra param (I believe there were discussions about adding it for some other runtime options).

3. For now, don't worry about it, and just add basic subclassing in constructor, while for static APIs libraries will have to continue using `setPrototypeOf` for wrapping resulting instances.

I don't have a strong opinion on this yet.

Huh, I wanted to say that for some reason no engine implements this subclassing and thought that these WebIDL steps must be explicitly linked from WebAssembly JS API spec, but just checked on macOS, and it seems JSC gets this right.

As such, it appears that this is indeed implementation bugs in SpiderMonkey and V8 which I'll report now.

Submitted.

Chromium issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1070695
Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1629998

@backes I don't suppose we need a separate issue to add tests; would you want to take care of a PR to add relevant spec tests and reference this issue?

I don't know much about Web IDL, and what exactly to test here. @Ms2ger, do you want to take this?

There's a simple test case attached to the pending patch at https://bugzilla.mozilla.org/show_bug.cgi?id=1629998

@lars-t-hansen That test looks simple yet robust enough. Would Mozilla want to upstream it to spec tests?

@RReverser, i'll look into it.

@RReverser That test case was upstreamed to WPT [1]

[1] web-platform-tests/wpt#23402

@eqrion Awesome, thanks!