Pick<TypeUnion, 'prop1' | 'prop2'> loses associations between 'prop1' and 'prop2'

[wwoods]

TypeScript Version: 3.7.0-dev.20190928

Search Terms: Pick, union bug, pick union bug

Code

interface A {
  type: 'A';
  inner: {
    a: boolean,
  };
  detail: string;
}

interface B {
  type: 'b';
  inner: {
    b: boolean,
  };
  detail: string;
}

type Combined = A | B;

// Broken - typescript does not raise an error, despite the mismatch between type 'A' and inner 'b'
type DescriptionBroken = Pick<Combined, 'type' | 'inner'>;
let obj: DescriptionBroken = {type: 'A', inner: {b: true}};

// OK - typescript raises an error.
type DescriptionOk = Pick<A, 'type' | 'inner'> | Pick<B, 'type' | 'inner'>;
let objOk: DescriptionOk = {type: 'A', inner: {b: true}};

Expected behavior:
The let obj line under the // Broken comment should raise an error, but does not.

Actual behavior:

The let obj line under the // Broken comment does not raise an error, presumably due to a disassociation between the 'type' and 'inner' elements.

Playground Link: https://www.typescriptlang.org/play/?ts=Nightly#code/JYOwLgpgTgZghgYwgAgILIN4ChnLATwAcIAuZAclXIG4dlQRoztdc4yAjAey4BsI4IADR0AvrVwATCGDjBeZAM5gooAOa1RWLKEixEKAEKY6BYmXIcadBkxOtkHTj36CRucXWmz5Sles1tMxQAYS4AWw5QCElkAF40ZAAfZENaLAB6DNSoLgBrCBBkAFo8IghFBFVCMGRJLgrkEC5aqDlFFEFkaFyoITqKwmBIPAALFHDgRXC4MARRxxkAdwhCsuIKKmRBWNsoCissYOQAEQqq4BrgLhBDXIKihIAFYAQ8gB4wyOjJfvJg8jJCh7cgAPlo-FqXA4ACsyGdKtUwNdbvc1gkMMELFR+ntmE48FAAK4QUSeTLZADyAGkSutzkjkG0po0uj0uFAAHRHcqnBmXZE3Sl5eLIF5vd6oP4AoHkEGgoHij6GaXlQEpOUgRhQMEQmTIaEw4Xw-lXIUijFYzbkXFauwYAkqElk6hAA

Related Issues: I found a number relating to Pick, but none with such a clean example or this exact issue. Maybe #33568?

[jack-williams]

Duplicate of #28339.

[wwoods]

The documentation for pick need to make that really, really clear. That is an extremely odd implementation detail. But, ok, closing this issue. Thanks for the link.