I couldn't find a relevant issue, but this does seem rather fundamental and against the principle of:

1. Statically identify constructs that are likely to be errors.

TypeScript Version:

1.8.7

Code

interface Optional {
    foo?: string;
}

function foo(o: Optional): void {}

foo('bar'); // should throw
foo(1); // should throw
foo({ bar: 'baz' }); // should (and does) throw

Expected behavior:

Expected behaviour is that you should be able to type guard against other non-objects and only accept Objects with no properties or declared properties.

Actual behavior:

Passing anything other than an object literal that contains extra properties is acceptable.