Assertion Functions cannot assert over private properties - unnecessary/wrong error message?

[phryneas]

Assertion Function "Private identifiers are not allowed outside class bodies."

Bug Report

🔎 Search Terms

assertion function, asserts, #, private members, Private identifiers are not allowed outside class bodies.

🕗 Version & Regression Information

This happens in 4.0.5, 4.4.3, 4.5.4 and the latest nightlies - so probably in all TS versions that support assertion functions and private class members.

⏯ Playground Link

A playground showcasing the problem

💻 Code

class Test {
    #ready: boolean = false;

    #assertReady(): asserts this is {
        // Private identifiers are not allowed outside class bodies.(18016)
        #ready: true // this is working perfectly fine though...
    } {
        if (this.#ready) {
            throw new Error("not ready")
        }
    }
}

🙁 Actual behavior

The code above works 100% as expected (the assertion function also narrows the type of this.#ready down to true correctly), but TS displays an error message:

Private identifiers are not allowed outside class bodies.(18016)

🙂 Expected behavior

The error message should be removed - this seems to work absolutely fine as it is. The error message seems to be a red herring.

[nmain]

The quick info on that function is a bit wonky:

(method) Test.#assertReady(): asserts this is {
    "__#46@#ready": true;
}

Where the number 46 goes up as you edit the document.

[RyanCavanaugh]

The error is intentional. While it may appear to work, this hasn't been designed or tested at all, so there might be dragons lurking, hard to say.

[MartinJohns]

@nmain #36548