Type narrowing not working for unions of tuples with object literals

It’s not possible to narrow unions of tuples with object literals

TypeScript Version: 3.4.5, 3.5.0-rc

Search Terms: tuple narrow object literals

Code:

Narrowing tuples with primitive types works as expected:

type X = ["a", "c"] | ["b", "b"]

function bar(x: X) {
  if (x[0] === "b") {
    return x[1] === "c" // correctly narrows to "c", shows "condition always false"
  }
}

However, when the tuple contains object literals, narrowing doesn’t work at all:

interface A { data: "a" }
interface B { data: "b" }
interface C { data: "c" }

type Y = [A, C] | [B, B]

function foo(y: Y) {
  if (y[0].data === "b") {
    return y[1].data === "c" // incorrectly narrows to "c" | "b"
  }
}

Expected behavior:

The object literal within the tuple is correctly narrowed down to type B

Actual behavior:

The object literal is not narrowed, leaving it at type C | B.

Playground Link: Link

Related Issues: #26323 #24120 #10530

[squidfunk]

3 years old, still not fixed. Is this issue dead? It was first scheduled for 3.7 and then repeatedly rescheduled until it ended in the backlog. Fixing this would help in several use cases and greatly improve expressivity. I have several parts in my code base where I need to fall back to very generic types that are not helpful at all what could be a union of tuples.

[Kingwl]

I’d like to try this if no one started work