default object parameter does not respect optional fields #5289

This typechecks:

function f({a = 1, b = 2}) {
}
f({});

But this does not:

function f({a = 1, b = 2} = {}) {
}
f();

with an error Type '{}' has no property 'a' and no string index.

I'd expect both to behave the same way.

This idiom is pretty convenient in ES6-style code and I don't see why a passed parameter would be treated differently from a default value here.