RSRP-466133: Incorrect handling of weak types

If I have a weak type (only optional fields) that also has a "wildcard" indexer (no matter if it is inherited or declared on the type itself), then I can't cast an object to the weak type, unless the object has any of optional fields declared in the weak type. It conflicts with the behavior of TS compiler, which allows such casts if we have "wildcard" indexer declared.

interface ITestA {
    [name: string]: any;  // This "wildcard" indexer is necessary for the cast to be legal
}

var a: ITestA = { a: "a", b: 3 }; // OK

interface ITestB extends ITestA {
    optionalField?: string;
}

var b: ITestB = { a: "a", b: 3 }; // Error: has no properties in common
var b2: ITestB = { a: "a", b: 3, optionalField: null }; // OK

This behavior appeared recently in version 2017.2. I guess after adding the support for TS 2.4. This is quite major, because now a lot of valid code gets rendered as invalid.