As mentioned in that issue, the editors suspect this was a refactoring issue, since it does not appear to ever actually have been possible for this to be *undefined*, and in any case is not assigned later. The only caller of ParseScript known to us is in HTML, which passes the realm of an environment settings object, which (I believe) cannot be *undefined*, so this restriction on types matches what the consumer actually passed.
Fixes https://github.com/tc39/ecma262/issues/3326.
As mentioned in that issue, the editors suspect this was a refactoring issue, since it does not appear to ever actually have been possible for this to be
*undefined*, and in any case is not assigned later. The only caller of ParseScript known to us is in HTML, which passes the realm of an environment settings object, which (I believe) cannot be*undefined*, so this restriction on types matches what the consumer actually passed.