@dcodeIO But it seems this changes in Binaryen produce some issues in features/reference-types test (instantiate error (debug)). Any ideas?

The error reads:

CompileError: WebAssembly.instantiate():
  Compiling function #8:"start:features/reference-types" failed:
    global.set[0] expected type externref, found global.get of type funcref @+929

What might have happened is that previously there was externref, which is disjoint from funcref, and their common supertype anyref, which hence can be assigned a funcref,

       anyref
       /   \
externref  funcref

but now there's just anyref = externref

  anyref  funcref

so a test case along the lines of

var ref: anyref = someFuncref

fails. Unfortunate that the design is going to be that an anyref cannot be assigned any ref, respectively that one can't pass a JS function into Wasm as an anyref (and call it without glue). Poor AS.

Relevant spec discussions:

• funcref <: anyref, or not? WebAssembly/gc#293
• anyref == externref introduces a typesystem mode WebAssembly/gc#296

Would you like to look into removing externref (respectively what was anyref) in the compiler as well (so assigning funcref for example won't fail), or would you prefer to keep this to a follow-up PR? I guess these can be split since the PR doesn't change the status quo as-is.

Yes, it is better to do it in a separate PR as it will be breaking changes

Unfortunate that the design is going to be that an anyref cannot be assigned any ref, respectively that one can't pass a JS function into Wasm as an anyref (and call it without glue).

I don't understand what you mean here. anyref is still the top reference type so it is able to hold funcref values and any other reference type value (at least unless we decide otherwise on WebAssembly/gc#293). Calling a JS function passed in as an anyref would require downcasting it to funcref so it can be called, but that should work.

Hey @tlively :) From discussion so far and reftypes history, I'd be surprised if funcref <: anyref remained, also since the major stakeholders appear to favor funcref </: anyref. If so, that'd leave us with an anyref that cannot be assigned any ref including what it entails. Is my expectation off?

If the result of the discussion is funcref </: anyref, then yes, that would be correct. I wouldn't be so sure of the result quite yet, though! If I had to guess, I'd say there's a 70% chance we keep funcref <: anyref given the info I have right now. Can you tell me a bit about how AS depends on or uses funcref <: anyref today?

That's good to hear! Apart from verifying assumptions in feature tests as of this issue, AS does not in practice depend on funcref <: anyref today. For a JS-inspired language with plans for seamless interop, a catch-all reference type as of anyref seems desirable, though, so for example any JS value can be piped through a Wasm export to a JS import that accepts any JS value. Being able to inspect, in just Wasm, that the anyref is a callable funcref, may similarly be handy.