Control flow doesn't recognise undefined check

[donaldpipowitch]

TypeScript Version: 2.1.1

Code

interface StdoutValue {
  stdout: string;
  stderr?: undefined;
}

interface StderrValue {
  stdout?: undefined;
  stderr: string;
}
type StreamValue = StdoutValue | StderrValue;

const stream: Array<StreamValue> = [
  { stdout: 'foo' },
  { stderr: 'bar' }
];

console.log(stream.map(value => value.stderr === undefined ? value.stdout.toUpperCase() : value.stderr.toUpperCase()));

Expected behavior:

No error with strict null checks.

Actual behavior:

Thinks value.stdout could be undefined on last line, but this is not possible.

[FranklinWhale]

Actually not only undefined check is not recognized, null check is not recognized either.

interface Properties {
	"property1"?: string
}
function getMap(): {[index: string]: Properties | null} {
	return {
		"1": {
			"property1": "test"
		},
		"2": null
	};
}
let map = getMap();
let item1Property1 = "1" in map && map["1"] !== null && "property1" in map["1"] ? map["1"].property1 : null;

The above code cannot be compiled when strictNullChecks is set to true. The following error is returned:

The right-hand side of an 'in' expression must be of type 'any', an object type or a type parameter

[ahejlsberg]

We currently consider a type a discriminated union only when all of the possible discriminant values have types composed from unit types. Effectively this means that the string and number cannot be part of the discriminant value type. It certainly would be possible to support combinations of unit types and infinite types, although it does add more complexity to the analysis.

[normalser]

Related: #12457

[RyanCavanaugh]

Tentative work item to investigate for the 2.4 milestone, approximately.

[eggers]

Here's another example of a failing case:

declare var maybeNull: number[] | undefined | null;

let isEmpty: boolean | undefined | null = maybeNull === undefined || maybeNull === null
    ? false
    : maybeNull; // Type of maybeNull should be `null | undefined`
// Instead error is:
//  Type 'false | number[]' is not assignable to type 'boolean'.
//  Type 'number[]' is not assignable to type 'boolean'.

[weswigham]

@eggers You just have your cases backwards:

declare var maybeNull: number[] | undefined | null;
let isEmpty: boolean | undefined | null = maybeNull === undefined || maybeNull === null
    ? maybeNull // Type of maybeNull _is_ `null | undefined`
    : false;

[ahejlsberg]

Closing as the original example is no longer an error.