ES6 won't destructure an object into pre-declared variables?

Question

This works:

const { foo, bar } = someFunc();

And so does this:

let { foo, bar } = someFunc();

But if I try to destructure into already declared variables...

let foo = 0;
let bar = 0;
{ foo, bar } = someFunc();

then:

Uncaught SyntaxError: Unexpected token =

Is this by design? Is there a workaround other than declaring a temp object to receive the value? I'm doing this in a switch/case statement; foo and bar are declared on top and used after the switch. So far I can only do:

const temp = someFunc();
foo = temp.foo;
bar = temp.bar;