Broken null checks for properties accessed via the brackets notation.

[kujon]

TypeScript Version: 2.3.2

Code

type MyType = {
    value?: string;
};

const obj: MyType = {
    value: '2017-05-04T13:18:21.203Z'
};

// -------------------------------

/**
 * Works!
 */
const a = obj.value ? new Date(obj.value) : null;

// -------------------------------

const VALUE = 'value';

/**
 * Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
 * Type 'undefined' is not assignable to type 'string'.
 */
const b = obj[VALUE] ? new Date(obj[VALUE]) : null;

// -------------------------------

/**
 * Works with a helper variable.
 */
const temp = obj[VALUE];

const c = temp ? new Date(temp) : null;

Expected behavior:
All 3 examples compile.

Actual behavior:

 Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
 Type 'undefined' is not assignable to type 'string'.

[RyanCavanaugh]

See #12636

[mhegazy]

this is actually a duplicate of #10530