Type mismatch error gets reported as an overload error

[jiribenes]

The following Effekt program:

def run(n: Int)    { action: => Unit }: Unit = action()
def run(s: String) { action: => Unit }: Unit = action()
def add(a: Int, b: Int): Int = a + b

def main() = {
  run(42) {
    val s: Int = add(1, "Hello world!");
    ()
  }
}

reports a type error in overloading of run:

Cannot typecheck call.
There are multiple overloads, which all fail to check:

Possible overload: module0::run of type (Int){() => Unit} => Unit
  Expected Int but got String. // <- the correct error

Possible overload: module0::run of type (String){() => Unit} => Unit
  Expected String but got Int.
  Expected Int but got String. // <- the correct error

However, the actual reported error should be that the second argument is of type String, whereas the expected type of the second parameter is Int. Fortunately, this error still gets reported at the site of the add call so the user can still technically figure this out.

---

If we combine multiple weird overloads:

def run(n: Int)    { action: => Unit }: Unit = action()
def run(s: String) { action: => Unit }: Unit = action()

def main() = {
  run(42) {
    "type error" ++ 42
  }
}

we get a way more confusing message, telling the user that the overload of run failed, but the problem is that the overload of ++ failed:

Cannot typecheck call.
There are multiple overloads, which all fail to check:

Possible overload: module0::run of type (Int){() => Unit} => Unit
  Expected String but got Int. // <- the correct error
  Expected Unit but got String.

Possible overload: module0::run of type (String){() => Unit} => Unit
  Expected String but got Int.  // <- a new error? (from the `++` operator)
  Expected String but got Int.  // <- the correct error
  Expected Unit but got String.