[js-api] ToWebAssemblyValue AO does not convert undefined to 0

[ADKaster]

In https://wpt.fyi/results/wasm/jsapi/global/constructor.any.html?label=experimental&label=master&aligned

Each major browser passes the subtests Explicit value undefined for type {f32, f64, i64}, which expect that an undefined value for value for each of those types will become the value 0.

However, https://webassembly.github.io/spec/js-api/#dom-global-global step 5 says to call ToWebAssemblyValue if and only if the value is missing, not if it is undefined. Thus we should be expecting a different result for the test cases:

• https://github.com/web-platform-tests/wpt/blob/master/wasm/jsapi/global/constructor.any.js#L123
• https://github.com/web-platform-tests/wpt/blob/master/wasm/jsapi/global/constructor.any.js#L148

Given that every major implementation is treating missing the same as undefined, the spec should probably change.

Otherwise, we would expect

new WebAssembly.Global( { "value": "i64" }, undefined); --> throws TypeError
new WebAssembly.Global({ "value": "f32", undefined); --> NaN
new WebAssembly.Global({ "value": "f64, undefined); --> NaN

As ToWebAssemblyValue calls https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tonumber and https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tobigint64 directly. Neither of those AOs in ES normalize undefined to 0.

[Ms2ger]

So to be clear, this is the test you're looking at:

const argument = { "value": "i32" };
const global = new WebAssembly.Global(argument, undefined);
assert_equals(global.value, 0, "value");
assert_equals(global.valueOf(), 0, "valueOf");

and the relevant idl is

[LegacyNamespace=WebAssembly, Exposed=*]
interface Global {
  constructor(GlobalDescriptor descriptor, optional any v);
  any valueOf();
  attribute any value;
};

The relevant prose is at step 1 of https://webidl.spec.whatwg.org/#create-an-interface-object

• args = [argument, undefined]
• n = 2

https://webidl.spec.whatwg.org/#compute-the-effective-overload-set

* F = { the single defined constructor } * maxarg = 2 * max = max(2, 2) = 2 * arguments = [GlobalDescriptor, optional any] * n = 2 * types = [GlobalDescriptor, any] * optionalityValues = ["required", "optional"] * S += (constructor, [GlobalDescriptor, any], ["required", "optional"]) * (i = 1) S += (constructor, [GlobalDescriptor], ["required"]) * (i = 0) break * S = [(constructor, [GlobalDescriptor, any], ["required", "optional"]), (constructor, [GlobalDescriptor], ["required"])]

• S = [(constructor, [GlobalDescriptor, any], ["required", "optional"]), (constructor, [GlobalDescriptor], ["required"])]

https://webidl.spec.whatwg.org/#dfn-overload-resolution-algorithm

• maxarg = 2
• n = 2
• argcount = 2
• S = [(constructor, [GlobalDescriptor, any], ["required", "optional"])]
• d = -1
• method = undefined
• values = []
• i = 0
• (i < d: false)
• (i = d: false)
• callable = constructor
• (i = d: false)
• i = 0:
  • V = argument
  • type = GlobalDescriptor
  • optionality = "required"
  • values += argument converted to a dictionary
• i = 1:
  • V = undefined
  • type = optional any
  • optionality = "optional"
  • values += "missing"

• <constructor, values> = <constructor, [argument converted to a dictionary, "missing"]

Now we move to the constructor steps at https://webassembly.github.io/spec/js-api/index.html#dom-global-global where step 4 checks v against the special value "missing" and calls DefaultValue(i32), i.e. i32.const 0.

No spec change needed.