Type guard does not work with object properties

[tkrotoff]

This works:

let strOrArray: Array<string> | string;
let str: string;
if (typeof strOrArray === 'string') {
  // No error
  str = strOrArray;
}

But not this:

interface Data {
  strOrArray: Array<string> | string;
}
let data: Data;
let str: string;
if (typeof data.strOrArray === 'string') {
  // Error:
  // Type 'string[] | string' is not assignable to type 'string'.
  // Type 'string[]' is not assignable to type 'string'.
  str = data.strOrArray;
}

Tested with tsc 1.7.5 and Visual Studio Code 0.10.6

[ahejlsberg]

This is a duplicate of #3812.