If statement variable reassignment fail type check

[dabreadman]

Bug Report

🔎 Search Terms

• if statement variable reassignment not assignable to parameter of type
• if statement variable reassignment not assignable
• if statement variable reassignment
• variable reassignment type
• variable reassignment

🕗 Version & Regression Information

• Relatively new to Typescript, encountered this on typescript@4.2.4

⏯ Playground Link

Playground link with relevant code

💻 Code

function fooBar(foo: "bar" | "baz"): boolean {
  return foo === "bar";
}

function fishTank(): boolean {
  let input = "";
  if (!input) {
    input = "bar";
  } else if (input !== "bar" && input !== "baz") {
    return false;
  }
  return fooBar(input);
}

🙁 Actual behavior

Failed to compile, Argument of type 'string' is not assignable to parameter of type '"bar" | "baz"'.ts(2345)

🙂 Expected behavior

Compiles successfully.

[DanielRosenwasser]

I believe we just don't narrow infinite types to singleton types in equality checks. @andrewbranch was dealing with a similar issue, but I can't recall what the reasoning was.

[andrewbranch]

It is kind of similar to #43584, but the premise of that issue is I feel like unknown may be worth treating specially. In this case, you probably just want a type annotation: https://www.typescriptlang.org/play?ts=4.3.0-dev.20210413#code/GYVwdgxgLglg9mABMOcBCBDATgChXALkQCIAjbYxAHxPIC9iBKI01AGwFMMkBvAKESIsHKCCxJ8iALwzaFANx8Avnz6hIsBMhgBnABYAVbgGsczRKziduifoM5REMMAAcQUImQrU5DH+AATDmBnDgDFQRhgRBwAQmc3KEZbAUEnV3dpOSxiCMQlRA42HQ4naJwEzNjZLxzEADJ69MTEaqlfJhS0oRExCQxijjyVVOFRcWRUTFxKpMURoA

[dabreadman]

I see, thanks.