Unions without discriminant properties do not perform excess property checking

TypeScript Version: 2.7.0-dev.201xxxxx

Code

// An object to hold all the possible options
type AllOptions = {
    startDate: Date
    endDate: Date
    author: number
}

// Any combination of startDate, endDate can be used
type DateOptions =
    | Pick<AllOptions, 'startDate'>
    | Pick<AllOptions, 'endDate'>
    | Pick<AllOptions, 'startDate' | 'endDate'>

type AuthorOptions = Pick<AllOptions, 'author'>

type AllowedOptions = DateOptions | AuthorOptions

const options: AllowedOptions = {
    startDate: new Date(),
    author: 1
}

Expected behavior:

An error that options cannot be coerced into type AllowedOptions because startDate and author cannot appear in the same object.

Actual behavior:

It compiles fine.

Start by reproducing the TypeScript example in the issue and checking whether the current compiler still accepts the object assigned to AllowedOptions. No source file or test is named; done means the compiler reports that startDate and author cannot appear together in the union value.