I don't know if 3.5 fixes this, but here's an example from 3.4

    function messUpTheArray(arr: Array<string | number>): void {
      arr.push(3);
    }

    const strings: Array<string> = ['foo', 'bar'];
    messUpTheArray(strings);

    const s: string = strings[2];
    console.log(s.toLowerCase())

You obviously can't push a number into an Array<String>, yet typescript allows it.

For my money, ReasonML is a better solution for type issues (hindley-milner types are awesome).