Confusing error message when value does not match union type

[Jessidhia]

TypeScript Version: nightly (2.2.0-dev.20161201)

Code

interface X<T extends string, U> {
    type: T
    data: U
}

type Y = X<'a', { a: string }>
type Z = X<'b', { b: string }>

type YZ = Y | Z

const y: YZ = {
    type: 'a',
    data: {}
}

Expected behavior:

Error message to show the union type, or at least the best partial match (type Y)

Actual behavior:

Error says the value of y does not match type Z.

[RyanCavanaugh]

Duplicate #6541