Typescript void generic problem

tsvoid.ts

interface Foo<T> {
  bar<T2>(callback:(t:T)=>T2) : T2;
}
var foo :Foo<string>;

// Works as expected.
foo.bar<number>((s:string) => {
  return 12345;
});

// Succeeds silently when it should actually fail...
foo.bar<void>((s:string) => {
  return 5;  // Not a void!!!
});

// Fails as expected.
foo.bar<number>((s:string) => {
});

// Fails as expected.
foo.bar((n:number) => {});