Destructuring for declaration prevents entangled type information / type implications

[ole-flavia]

TypeScript Version: 3.8.0-dev.20191022

Search Terms: destructuring

Code

type X = {x: true, y: number[]};
type Y = {x: false, y: undefined};
type Z = X | Y;

function a(x: Z) {
  if (x.x) {
    x.y.forEach(console.log);
  }
}

function b({x, y}: Z) {
  if (x) {
    y.forEach(console.log);
 // ^ Object is possibly 'undefined'
  }
}

Expected behavior:
Function b should compile.

Actual behavior:
Typescript does not detect any relation between x and y variables in function b, so it does not figure out that x === true => y is number[].

Playground Link: http://www.typescriptlang.org/play/?ts=3.8.0-dev.20191022&ssl=1&ssc=1&pln=17&pc=1#code/C4TwDgpgBAGlC8UDeAPAXFYAnArhANFCBgHY4C2ARhFgNoC6AvgNwBQokUAmgsulADMAhgBsAzgSIYcJACYQBASxIRZLduGgAtXnAA+3NqwEyAxsEUB7ElCEAKfloCUyVlCiKBUBwDoULpDd3KBQfEB8BSywAUSFTAAs7U2sxSxEIHxFLAHMnNndGVkLjMwtrKEo7VEIQRgxnV3dPb39G4PDImLjE5JJU9MycvLcAehGoAD0oAHlKACsIcw8xKDBLMTFFShEQKAByGXklFVk9oMLCoA

Related Issues: None

[jack-williams]

Duplicate of #32639, and tracked at #12184.