Type guards don't work on key/value objects

[diestrin]

TypeScript Version: 2.2.2 with --strictNullChecks

Code

let test: string;

const obj1: {val: string|number} = {val: ''};
if (typeof obj1.val === 'string') {
  test = obj1.val; // => Works fine
}

const obj2: {[k: string]: string|number} = {val: ''};
if (typeof obj2.val === 'string') {
  test = obj2.val; // => Type 'string | number' is not assignable to type 'string'
}

Expected behavior:
Type guards to work on key/value objects

Actual behavior:
Can't infer the correct type after check

[RyanCavanaugh]

Interesting. This will likely require some architectural changes as we currently have no symbol to associate with the .val property access and thus nothing to narrow by.

[RyanCavanaugh]

This works now