TypeScript Version: 2.4.2

Code

class A {
  constructor(); // Error: overload signature not compatible with impl
  constructor(a: number | undefined) {}
}

class A {
  constructor();
  constructor(a?: number) {} // this one works
}

Expected behavior:
I expected the first example to work especially since the type of parameter a is identical in either constructor implementation. I think it is helpful when writing overloads to not use optional parameters in the implementation signature and be explicit about the types that might exist in each position.

Actual behavior:
The first results in the noted error.

Commentary:
The following error may a moral equivalent of the error I am objecting to:

function foo(a: number | undefined) { }
foo(); // error, even though it's "implicitly" passing undefined

but I think this case is different as on the call side the error is justifiable on the grounds that we think it's likely the caller forgot a parameter (even if undefined is an allowable type, you should be explicit).