Implementation signatures that do not contain all overloaded signatures are allowed in class definitions

🔎 Search Terms

implementation contain overloaded class member

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about overloading class member functions

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/FAYwNghgzlAEDCwDexZtgMwPZYBSwgH5YAuWAOwFcBbAIwFMAnWASlNgDcsBLAEwG5U6bHgLsqdJgBpYtcTQbM2ZLn0HpMOfBHmTGM2sTITFrdqt5C0KDRpBZyULGHoA6MFgDm+AC4BPAAd6LAwxFn5YAHpI2H8gkLFuOBMmK1gAX2BM4HJ6AHcEXBZXESKIqJiAxm5yHzgAIkpyXnoMGvpeeuAgA

💻 Code

class C
{
    foo( a? : number ) : void;
    foo( a : number, b : number ) : void;
    foo( a : number, b? : number ) : void
    {
        console.log( typeof a ); // typeof a is number
    }
}

new C().foo();  // prints "undefined"

🙁 Actual behavior

The compilation succeeded.

The type of argument "a" in function foo is number but prints "undefined" which is wrong.

Because the implementation signature of foo does not include all overload signatures, so compilation should fail.

🙂 Expected behavior

The compilation should fail.

Additional information about the issue

This bug is only reproduced with class member functions.

Start with the linked TypeScript Playground reproduction and confirm that the class-member overloads compile despite the implementation signature omitting the no-argument case. Trace the class member overload-checking path, add a regression test for this example, and verify that compilation now fails as expected.