Control flow analysis for dependent parameters doesn't work for methods

[jcalz]

Bug Report

🔎 Search Terms

control flow analysis, dependent parameters, methods, #47190, #48110

🕗 Version & Regression Information

This behavior has been present since #47190 was merged, so TS4.6.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
  method(...args: ['a', number] | ['b', string]): void
};

const methodSyntax: Foo = {
  method(kind, payload) {
    if (kind === 'a') {
      payload.toFixed(); // error, Property 'toFixed' does not exist on type 'string | number'
    }
    if (kind === 'b') {
      payload.toUpperCase(); // error, Property 'toUpperCase' does not exist on type 'string | number'
    }
  }
}

🙁 Actual behavior

Checking kind inside the body of method() does not narrow payload from string | number.

🙂 Expected behavior

Given #47190, I'd expect kind === 'a' to narrow payload to number, and kind === 'b' to narrow payload to string, which does happen when method is a function expression:

const funcExpr: Foo = {
  method: function (kind, payload) {
    if (kind === 'a') {
      payload.toFixed()  // okay
    }
    if (kind === 'b') {
      payload.toUpperCase(); // okay
    }
  }
}

And when method is an arrow function:

const arrowFunc: Foo = {
  method: (kind, payload) => {
    if (kind === 'a') {
      payload.toFixed(); // okay
    }
    if (kind === 'b') {
      payload.toUpperCase(); // okay
    }
  }
}

And by #46266 when destructured from a rest argument:

const manualDestructuring: Foo = {
  method(...args) {
    const [kind, payload] = args;
    if (kind === 'a') {
      payload.toFixed(); // okay
    }
    if (kind === 'b') {
      payload.toUpperCase(); // okay
    }
  }
} 

I don't know whether this is really a bug or just a follow-up feature request to add to #46658 / #47190. For context: I mentioned this in #47190, and Mateusz Burzyński (@Andarist) has written a PR for it at #48110.

[RyanCavanaugh]

Seems like an oversight