Unions accept extra keys with incorrect properties

[luke-john]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)

Code

interface SinglePropA { a: 'test-a' }
interface SinglePropB { b: 'test-b' }
type AorB =
    | SinglePropA
    | SinglePropB

const testAandB: AorB = {
    a: 'test-a',
    b: 'test-bs'
} // passes on 2.4.1 and 2.5.0-dev.20170629 

const testAandC: AorB = {
    a: 'test-a',
    c: 'test-b'
} // fails on 2.4.1 and 2.5.0-dev.20170629
/*
    Object literal may only specify known properties,
    and 'c' does not exist in type 'SubsetAnimal'.
*/

typescript playground example

Expected behavior:

Unions don't accept extra properties where the key matches another member of the union.

Actual behavior:

Unions accept extra properties where the key matches another member of the union but the value is incorrect.

Note. Came across this when trying to find a workaround for the following issue #17003

[RyanCavanaugh]

See discussion in #16679

[luke-john]

Thanks for that.