TypeScript Version: 2.3.4 and current playground

Code

namespace ns {
    export default class Foo{ } // no error
}
ns.Foo; // error
ns.default; // no error

playground

Expected behavior:
Error at declaration that default exports are only allowed in ECMAScript modules.

Actual behavior:
No error at declaration.

Code completion shows ns.Foo but compiler prints warning.
Code completion does not show ns.default but tooltip reveals it is class ns.Foo. That's completely wrong because the emit does not contain any default export.