Type guards / excess object literals property checking fails with Object.assign()

[18steps]

Object spread is fixed in the nightly build, as per #13878 last commented on 2 days ago, but Object.assign() is not fixed, and it doesn't seem to be mentioned in any existing issues.

TypeScript Version: 2.3.2, 2.4.0-dev.20170519

Code

interface Limited {
  property: string;
}

const assignment = (): Limited => {
  return {
    property: 'this is fine'
  };
};

const assignment2 = (): Limited => {
  return {
    property: 'this is fine',
    property2: 'this is not fine, and shouldn\'t be',
  };
};

const nothing = {};
const somethingElse = {
  something: 'something'
};
const existing = {
  property: 'existing'
};

const assignment3 = (): Limited => {
  return {
    ...nothing,
    property2: 'this is not fine, and shouldn\'t be',
  };
};

const assignment4 = (): Limited => {
  return {
    ...somethingElse,
    property2: 'this is not fine, and shouldn\'t be',
  };
};

// - fails in 2.3.2 but not 2.4.0-dev.20170519
const assignment5 = (): Limited => {
  return {
    ...existing,
    property2: 'this should not be fine, and is not in 2.4.0-dev.20170519',
  };
};

// !
const assignment6 = (): Limited => {
  return Object.assign(existing, {
    property2: 'this should not be fine either, but is',
  });
};

Expected behavior:
Type guards should produce errors when attempting to assign a property not defined in the interface.

assignment5() and assignment6() should produce errors.

Actual behavior:
With Object.assign() or the Object spread operator, type guards fail to produce an error when attempting to assign a property not defined in the interface, as long as the required properties exist.

assignment5() and assignment6() does not produce an error.

[mhegazy]

this is tracked by #11100

Object spread is fixed in the nightly build, as per #13878 last commented on 2 days ago, but Object.assign() is not fixed, and it doesn't seem to be mentioned in any existing issues.

#13878 is a different issue than what you are reporting.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.