Inverted union type narrowing was broken in 4.9

🔎 Search Terms

"union type", "narrowing", "inverted", "4.9"

🕗 Version & Regression Information

• This changed between versions 4.8.4 and 4.9.5 (probably caused by https://github.com/microsoft/TypeScript/pull/50397?)

⏯ Playground Link

Playground link

💻 Code

interface Foo { foo: string; }

interface Bar { bar: string; }

interface Baz { baz: string; }

function myFunction(input: Foo | Bar | Baz) {
    const isBaz = 'baz' in input;
    const isBar = 
      //'bar' in input; // This works to narrow `bar` when true and narrow `foo` in the else block
      !('foo' in input) && !isBaz; // This works to narrow `bar` when true but does not narrow `foo` in the else block
    let x: string;
    if (isBaz) {
      x = input.baz;
    } else if (isBar) {
      x = input.bar;
    } else {
      // In 4.8.4 and earlier, `input` is narrowed to `Foo`
      // In 4.9.5 and later, `input` is narrowed to `Foo | ((Foo | Bar) & Record<"baz", unknown>)`
      x = input.foo;
    }
}

🙁 Actual behavior

In 4.9.5 and later, in the else block, input.foo incorrectly narrows and errors

🙂 Expected behavior

In 4.8.4 and earlier, in the else block, input.foo correctly narrows

Additional information about the issue

No response

Start with the supplied Playground link and reproduce the difference between TypeScript 4.8.4 and 4.9.5 using the example in the issue. Trace the type narrowing for the inverted union checks in the type checker. Done means the else branch narrows input to Foo and accepts input.foo without an error in the affected versions.