assertion signature with primitive union: resulting type too narrow

[NaridaL]

TypeScript Version: 3.7.5, 3.8.0-dev.20200201

Search Terms:
assertion signature

Code

interface Foo {
    action(): void
    bar?: string
}

declare function assertFooBar(foo: any): asserts foo is ((Foo & { bar: string })|true)

declare const x: Foo | true
assertFooBar(x)

const y = x

Expected behavior:
y has type (Foo & { bar: string }) | true

Actual behavior:
y has type true.

Playground Link:
https://www.typescriptlang.org/play/?ts=3.8.0-dev.20200201&ssl=11&ssc=12&pln=1&pc=1#code/JYOwLgpgTgZghgYwgAgGIHt3IN4ChkHKJjDogAUAlAFzIBu6wAJvoQEZxQD8tAzmFFABzXAF9cuJhAQAbTihgBXEAhJkivXtDAZ0AIU7kYmWnBABPGhq1QwvZMazB75cruQAyHMg5Q+A4WRRSgAfAUUISgkpWXlkBDJ+ZAAPWncQ5HCIXDhNbV0DKHJkqNwEkCTzZABeFIkwcwAHFAAVGuRXN0xPb19-QRAhIMpkDKzKEa8urDGoCNKG5uQADXbdAHVgMAALQoAedMy5iAA+ZHqmlA2t3c49lrPatohkyBAme3cuNG6vbB9OP1AqJkLQWrhFldMJsdoUAIIWNbQm77MzmE64IA
Related Issues:

[RyanCavanaugh]

Duplicate of #35953

[NaridaL]

@RyanCavanaugh Yeah, I was half expecting this to be by design due to some type system subtlety. Thanks in any case.