Truncation in "convert a Web IDL arguments list to an ECMAScript arguments list" seems (potentially) incorrect

[Ms2ger]

What is the issue with the Web IDL Standard?

https://webidl.spec.whatwg.org/#web-idl-arguments-list-converting

1. Let esArgs be an empty list.

2. Let i be 0.

3. Let count be 0.

4. While i < args’s size:

   1. If args[i] is the special value “missing”, then append undefined to esArgs.

   2. Otherwise, args[i] is an IDL value:

      1. Let convertResult be the result of converting args[i] to an ECMAScript value. Rethrow any exceptions.

      2. Append convertResult to esArgs.

      3. Set count to i + 1.

   3. Set i to i + 1.

5. Truncate esArgs to contain count items.

6. Return esArgs.

As far as I can tell, args could be « value, "missing", optionalValue », in which case count would be 2 in step 5 and the output would be « value, undefined ». This seems incorrect.

If that case is impossible, the spec should say that. Otherwise I suppose it should count the number of trailing "missing" values in args separately.