Reported by @wycats. Compile this under strictNullChecks and noImplicitAny.

declare function someAbstraction(x: any): any;

let x: string;

someAbstraction(() => {
  x = "hello";
});

// All of the following are errors.
x;
x as any;
x!;

I'd argue that both type assertions should be able to signal to the type system that the variable has been initialized.