Type narrowing not scope aware

[ithinkihaveacat]

TypeScript Version

2.0 (installed via npm install typescript@2)

Code

function foo(x: number|string): number {

  if (typeof x === "string") {
    x = 1;
  }

  function bar() {
    return 1 + x;
  }

  return bar();
}

Expected behavior

Code compiles cleanly.

Actual behavior

Compiler complains Operator '+' cannot be applied to types 'number' and 'number | string' (within function bar()).

[JannesMeyer]

I am just a TypeScript user myself, but this is much more complex than you might think, because function bar is hoisted to the top of the scope before your if:

function foo(x: number|string): number {

  function bar() {
    return 1 + x;
  }

  if (typeof x === "string") {
    x = 1;
  }

  return bar();
}

[ithinkihaveacat]

Yes, true. I realize this is a slightly tricky case, but I think this type error/warning is bogus: x can only be a number within bar(), and I got the impression from the docs that TS 2.0 (though not 1.8) is supposed to know this.

[yortus]

TypeScript does not reach that level of control flow analysis (yet). See also #7719 and #10180.