Try to fix #926 by providing more information about type variable origins

[jiribenes]

I quickly hacked together a prototype of a way to resolve #926.
Please, feel free to assign yourself and clean this up. :)

Example:

type Result[T, E] {
  Success(value: T)
  Failure(errors: List[E])
}

def addError[T, E](result: Result[T, E], e: E) =
  result match {
    case Success(value)  => Failure([e])
    case Failure(errors) => Failure(Cons(e, errors))
  }

def main() = ()

now reports:

[error] ambig.effekt:6:1: Cannot fully infer type for addError: [T, E](Result[T, E], E) => Result[T, E], because the following type variables are ambiguous: T from Call(IdTarget(IdRef(List(),Failure)),List(),List(Call(IdTarget(IdRef(List(),Cons)),List(),List(Var(IdRef(List(),e)), Call(IdTarget(IdRef(List(),Nil)),List(),List(),List())),List())),List())
def addError[T, E](result: Result[T, E], e: E) =
^
[error] ambig.effekt:8:29: Cannot infer type argument T of IdTarget(IdRef(List(),Failure)), maybe consider annotating it?
    case Success(value)  => Failure([e])
                            ^^^^^^^^^^^^
[error] ambig.effekt:9:29: Cannot infer type argument T of IdTarget(IdRef(List(),Failure)), maybe consider annotating it?
    case Failure(errors) => Failure(Cons(e, errors))
                            ^^^^^^^^^^^^^^^^^^^^^^^^

---

What I'd reasonably want it to report is:

[error] ambig.effekt:6:1: Cannot fully infer type for addError: [T, E](Result[T, E], E) => Result[T, E], because the following type variables are ambiguous: T from Failure[T, E]
def addError[T, E](result: Result[T, E], e: E) =
^
[error] ambig.effekt:8:29: Cannot infer type argument T of Failure[T, E], maybe consider annotating it?
    case Success(value)  => Failure([e])
                            ^^^^^^^^^^^^
[error] ambig.effekt:9:29: Cannot infer type argument T of Failure[T, E], maybe consider annotating it?
    case Failure(errors) => Failure(Cons(e, errors))
                            ^^^^^^^^^^^^^^^^^^^^^^^^

My ideal error message would be:

[error] ambig.effekt:6:1: Cannot fully infer type for addError: [T, E](Result[T, E], E) => Result[T, E], because of ambiguous type arguments.
def addError[T, E](result: Result[T, E], e: E) =
^
    [note] ambig.effekt:8:29: Cannot infer type argument T of Failure[T, E] used here, consider annotating it.
        case Success(value)  => Failure([e])
                                ^^^^^^^^^^^^
    [note] ambig.effekt:9:29: Cannot infer type argument T of Failure[T, E] used here, consider annotating it.
        case Failure(errors) => Failure(Cons(e, errors))
                                ^^^^^^^^^^^^^^^^^^^^^^^^

---

TODOs:

• get symbol from the call tree to pretty-print origin of the type variable
• clean up

[jiribenes]

Nobody seems interested in finishing this and it's currently not a priority, so I'm closing the PR. Feel free to continue with this if you want to though.