Misleading error message when mutable variable shadows outer function

[jiribenes]

The following code has a somewhat misleading error message: there is a sum which is mutable, it's just not something we can reach:

The code in question

def sum(xs: List[Int]): Int = {
  var sum = xs.foreach { x =>
    sum = sum + x
    //  ^ ~~~
  }
  sum
}

Of course when you use res for the mutable variable and keep sum for the function, you get Could not resolve term res, which is probably a better message in this case.

I have no real use case for this, I just introduced a typo by accident in https://effekt-lang.org/tour/variables and then wondered for a brief moment.

---

Here's the relevant compiler code:

effekt/effekt/shared/src/main/scala/effekt/Namer.scala

Lines 516 to 521 in afb9ef0

	case source.Assign(id, expr) => Context.resolveVar(id) match {
	case _: VarBinder | _: RegBinder => resolveGeneric(expr)
	case _: ValBinder | _: ValueParam => Context.abort(pretty"Can only assign to mutable variables, but ${id.name} is a constant.")
	case y: Wildcard => Context.abort(s"Trying to assign to a wildcard, which is not allowed.")
	case _ => Context.abort(s"Can only assign to mutable variables.")
	}