[js-api] Ambiguity in WebAssembly.instance() importObject requirement with JS builtins imports

[SamVerschueren]

The WebAssembly JS-API specification currently states that WebAssembly.Instance() must throw if importObject is undefined while the module declares imports. However, modules that only import JS builtins (for example from the wasm:js-string namespace) do not require an importObject.

The spec text does not clearly define this exception, which leads to inconsistent behavior across implementations. In particular, Firefox currently throws a TypeError in this situation, while Chrome (and I believe Safari) do not.

Currently the spec states

If module.imports is not empty, and importObject is undefined, throw a TypeError exception.

I think the spec should explicitly state that imports satisfied by builtins such as wasm:js-string do not require a user-supplied importObject, even if module.imports is non-empty.

[eqrion]

The spec text does not clearly define this exception, which leads to inconsistent behavior across implementations. In particular, Firefox currently throws a TypeError in this situation, while Chrome (and I believe Safari) do not.

To be a little pedantic, the spec is unambiguous that throwing a TypeError in this case is required.

But I do agree that is too strict and should be relaxed. It'll require re-ordering the steps of "read the imports" a bit, but I think it can be done.