Property goes undetermined on discriminated union

[saschanaz]

TypeScript Version: 2.0 beta

Code

"use strict";

type IDLMemberTypes = OperationMemberType | ConstantMemberType;

interface IDLTypeDescription {
    origin: string;
}

interface InterfaceType {
    members: IDLMemberTypes[];
}

interface OperationMemberType {
    type: "operation";
    idlType: IDLTypeDescription | null;
}

interface ConstantMemberType {
    type: "const";
    idlType: string;
}

// case 1
function insertInterface(callbackType: InterfaceType) {
    for (const memberType of callbackType.members) {
        if (memberType.type === "const") {
            memberType.idlType
            // type of idlType: string, no error
        }
        else if (memberType.type === "operation") {
            memberType.idlType.origin;
            // type of idlType: IDLTypeDescription at least on mouse hover
            /*
            Mutually incompatible messages appear simultaneously:

            1. (property) IDLTypeDescription.origin: string
            2. Property 'origin' does not exist on type 'string'
            */
            (memberType.idlType as IDLTypeDescription);
            // Error: Type 'string' cannot be converted to type 'IDLTypeDescription'
        }
    }
}

// case 2
function insertInterface2(callbackType: InterfaceType) {
    for (const memberType of callbackType.members) {
        if (memberType.type === "operation") {
            memberType.idlType.origin
            // type of idlType: IDLTypeDescription
            // type of origin: string, no error
        }
    }
}

// case 3
function foo(memberType: IDLMemberTypes) {
    if (memberType.type === "const") {
        memberType.idlType
        // type of idlType: string, no error
    }
    else if (memberType.type === "operation") {
        memberType.idlType.origin
        // type of idlType: IDLTypeDescription
        // type of origin: string, no error
    }
}

Expected behavior:

memberType.idlType.origin should be string in both case thanks to the type guard

Actual behavior:

Type guard confuses the type of idlType