cannot coerce undefined to object! in template literal

[dckc]

The template literal expression here throws; is that by design? I'm looking thru various specs and docs and stuff and I can't quite tell whether this OK. The diagnostic is misleading, since neither Object(undefined) nor String(undefined) throws.

export default function main() {
  const undef = undefined;
  const o = Object(undefined);
  const s0 = String(undefined);
  const s = `undef:${undef}`;
  trace(s + '\n');
}

If I try to make my own template tag, the 2nd arg seems to get undefined rather than an array of values. The exception comes on the parts.push line, presumably when evaluating args[i - 1]:

export default function main() {
  const undef = undefined;
  const o = Object(undefined);
  const s0 = String(undefined);
  const s = allstr`undef:${undef}`;
  trace(s + '\n');
}


function allstr(quasis, args) {
  const parts = [quasis[0]];
  for (let i = 1; i < quasis.length; i += 1) {
    parts.push(String(args[i - 1]), quasis[i]);
  }
  return parts.join('');
}

context:
https://github.com/Agoric/SwingSet/blob/a74f608035bbb33f7cc6846c69fd16993131bc48/src/kernel/liveSlots.js#L150

[phoddie]

Thank you for the report.

XS does behave differently than the engines in browsers for this case. The exception you see os generated when trying to call toString on undefined. The exception is correct when calling toString on undefined, but it appears that toString should not be invoked in that case.

It looks like the issue hinges on the notes in 12.2.9.6:

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

[erights]

The important part of that is it bottoms out in the behavior of the internal ToString function
https://tc39.es/ecma262/#sec-tostring

[phoddie]

The important part of that is it bottoms out in the behavior of the internal ToString function

@erights - yes, exactly. I didn't have it quite right above. Patrick noted that XS took a short cut on this step, by trying to call the objects's toString which is not quite ToString.

6. Let last be ? ToString(sub).

@dckc - A fix is committed. The change adds a new opcode making it necessary to rebuild tools as well as projects.

[dckc]

I can confirm that the undef:${undef} problem is fixed in 54361c6 .

The allstr template tag symptoms persist, but that's not blocking my work; IOU a separate issue.