Using spread operator breaks typechecking

[sh-rp]

My apologies if this has been reported before, I have done a search of the issues, and with the keyword spread this has not come up. I also believe that this functionality worked in TS 2.1 but is now broken.

TypeScript Version: 2.3.2

Code

interface ICar {
  wheels: number
  brand: string
}

let car:ICar = {
  wheels: 4,
  brand: 'volvo'
}

car = {
  ...car,
  legs: 5 // this should not work
}

Expected behavior:
Compile error when trying to add the legs attribute to a variable of type ICar.

Actual behavior:
Will compile and add the faulty attribute

[mhegazy]

We did discuss this in #14853 and the decision was to flag these as errors.

[sh-rp]

Hey @mhegazy , thanks for getting back to me so promptly. Great to hear that it is in the roadmap.

[sandersn]

My old PR fixes this issue as well, so I re-opened it and will merge it after I get a review.