Truthiness narrowing does not work as expected with &&

[Juneezee]

Bug Report

🔎 Search Terms

• Truthiness narrowing
• Truthiness narrowing with &&
• Truthiness narrowing with other conditional expressions
• Narrowing
• Control flow analysis

🕗 Version & Regression Information

Possibly related to #52272

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Circle {
  kind: "circle";
  x: number;
}
 
interface Square {
  kind: "square";
  x: number | null;
}

type Shape = Circle | Square;

function fn1(shape: Shape) {
  if (shape.kind === "square" && shape.x === null) {
    const { kind, x } = shape // { kind: "square", x: number } as expected

    return
  }

  // After the conditional branch above, we know that `shape.x` can no longer
  // be `null`.

  if (shape.kind === "square") {
    const { kind, x } = shape // { kind: "square", x: number | null }
                              // type of `x` should be `number` only
  }

  const newKind = shape.kind // type is `"circle" | "square"` as expected
  const newX = shape.x       // type of `shape.x` should be narrowed to `number` only
}

function fn2(shape: Shape) {
  // it works when we narrow the type without other conditonal expressions
  if (shape.x === null) {
    const { kind, x } = shape // { kind: "square", x: number } as expected

    return
  }

  if (shape.kind === "square") {
    const { kind, x } = shape // { kind: "square", x: number } as expected
  }

  const newKind = shape.kind // type is `"circle" | "square"` as expected
  const newX = shape.x // type is `number` only as expected
}

🙁 Actual behavior

TypeScript fails to narrow the type of shape.x to number only in fn1 after the first conditional branch if (shape.kind === "square" && shape.x === null) 😕

🙂 Expected behavior

After the first conditional branch if (shape.kind === "square" && shape.x === null) in fn1, shape.x can not be null anymore.

However, doing just if (shape.x === null) in fn2 works fine

[fatcerberus]

After the first check, shape.x might still be null, if shape.kind is something other than square. While that possibility is precluded by the second check for shape.kind === "square", the compiler isn't capable of counterfactual reasoning like "X or Y; X is false, therefore Y". At any given point, type narrowing either happens or it doesn't and subsequent checks can only work from there.

[RyanCavanaugh]

the compiler isn't capable of counterfactual reasoning like "X or Y; X is false, therefore Y"

I've been trying to state this in a reasonable way for years and you finally provided a phrasing that makes sense 💖

[guilhemheinrich]

I posted a similar issue here: https://stackoverflow.com/questions/78287716/is-there-an-explanation-to-the-following-typescript-snippet-not-correctly-inferr