Normative: only coerce once in BigInt constructor

[bakkot]

(This was raised on the discourse.)

Consider

let first = true;
let v = {
  [Symbol.toPrimitive]() {
    console.log('called');
    if (first) {
      first = false;
      return 'string';
    } else {
      return 0n;
    }
  }
};
BigInt(v);

Per spec this should print called twice and complete normally - step 3 of the BigInt constructor will observe that the first call to ToPrimitive returns a value which is not a number and then step 4 will invoke ToBigInt on the original, pre-coercion value, which will invoke Symbol.toPrimitive again.

In V8, SpiderMonkey, JavaScriptCore, and XS, this code will in fact only print called once and then throw a TypeError because the initial 'string' can't be coerced to a BigInt - i.e., in step 4 they use the post-coercion value, not the pre-coercion value. That's the behavior proposed in this PR.

GraalJS and Engine262 implement the spec faithfully.

[syg]

haha oh no

[apaprocki]

@bakkot So Date constructor 4c is basically the same issue?

let first = true;
let v = {
  [Symbol.toPrimitive]() {
    console.log('called');
    if (first) {
      first = false;
      return 0;
    } else {
      return 'string';
    }
  }
};
new Date(v);

[Log] called
< Wed Dec 31 1969 19:00:00 GMT-0500 (EST)

[bakkot]

@apaprocki The Date constructor has a similar pattern but is currently specified correctly: it does Let v be ? ToPrimitive(value). [...] Let tv be ? ToNumber(v)., i.e., the second line uses the post-coercion value instead of the pre-coercion value.

(That second line does want a suppress-effects="user-code", though.)

[apaprocki]

Ah, my brain read what you wrote (without seeing the 1-line diff) and thought this was to fix behavior to match the spec as opposed to the other way around. Thanks!

[littledan]

Wow thanks for the fix 😳

[bakkot]

Tests: tc39/test262#3631