Type Gaurds Don't Narrow Type in Inner Functions

[ethanresnick]

TypeScript Version: 2.0.2

Code

  function isNumber(test: number|string): test is number {
    return typeof test === "number";
  }
  function blah(arg: () => void): void { }

  let x: any = 4;

  if(isNumber(x)) {
    x; // x is correctly narrowed to number here
    blah(() => {
      x; // typeof x is any again
    })
  }

Expected behavior:
(I think) x should still be inferred as type number in the inner function.

Actual behavior:
x is inferred as any

[normalser]

#7719 (comment)

[ahejlsberg]

FYI, it works as expected if you use const instead of let because the compiler then knows that x will never be reassigned.

[ethanresnick]

Got it. Thanks!