We currently allow IDL files to specify [optional,default(foo)], but without the default() bit the resulting generated .h file doesn't specify a default, so C++ consumers have to provide the optional argument, which is unnecessarily verbose.

Where the default(x) annotation is not provided, we should create one for the relevant argument -- false for bool, 0 for uint, nullptr for interfaces/objects and jsval.

There's less of a point doing this for non-void methods, as the resulting header will have a non-optional final _retval argument without a default value.

Trivial searching suggests there are at least 16 methods that would benefit: https://searchfox.org/mozilla-central/search?q=void+.*+%5C%5Boptional&path=idl&case=false&regexp=true