Fix backtracking in nested overloads (#1007)

Fixes #996.

Author: marzipankaiser

effekt/shared/src/main/scala/effekt/Typer.scala

@@ -1179,7 +1179,7 @@ object Typer extends Phase[NameResolved, Typechecked] {
         // use the typer state after this checking pass
         Context.restoreTyperstate(st)
         // reassign symbol of fun to resolved calltarget symbol
-        Context.assignSymbol(id, sym)
+        Context.annotateSymbol(id, sym)
 
         return tpe
 
@@ -1607,6 +1607,12 @@ trait TyperOps extends ContextOps { self: Context =>
       }
     }
 
+  private[typer] def annotateSymbol(id: source.IdRef, sym: symbols.Symbol) =
+    annotations.update(Annotations.Symbol, id, sym)
+
+  override def symbolOption(id: source.Id): Option[Symbol] =
+    Context.annotations.get(Annotations.Symbol, id).orElse { super.symbolOption(id) }
+
   private[typer] def bind(s: ValueSymbol, tpe: ValueType): Unit =
     annotations.update(Annotations.ValueType, s, tpe)
 
@@ -1702,6 +1708,8 @@ trait TyperOps extends ContextOps { self: Context =>
     annotations.updateAndCommit(Annotations.BoundCapabilities) { case (t, caps) => caps }
     annotations.updateAndCommit(Annotations.CapabilityArguments) { case (t, caps) => caps }
     annotations.updateAndCommit(Annotations.CapabilityReceiver) { case (t, caps) => caps }
+
+    annotations.updateAndCommit(Annotations.Symbol) { case (t, sym) => sym }
   }
 
   //</editor-fold>

effekt/shared/src/main/scala/effekt/context/Annotations.scala

@@ -467,7 +467,7 @@ trait TreeAnnotations { self: Context =>
       // addDefinedSymbolToSource(sym)
   }
 
-  def symbolOf(id: source.Id): Symbol = symbolOption(id) getOrElse {
+  def symbolOf(id: source.Id): Symbol = this.symbolOption(id) getOrElse {
     panic(s"Internal Compiler Error: Cannot find symbol for ${id}")
   }
   def symbolOption(id: source.Id): Option[Symbol] =

examples/pos/imports.effekt

@@ -3,7 +3,7 @@ module imports
 import examples/pos/lists
 
 def main() = {
-  val l = Cons(1, Cons(2, Nil()));
+  val l: lists::List[Int] = Cons(1, Cons(2, Nil()));
   foreach(l) { el => println(el) };
   ()
 }

examples/pos/issue996.check

@@ -0,0 +1,17 @@
+extern type BitArray
+type IntArray = Unit
+
+def nums(): IntArray = ()
+
+def for(arr: BitArray) { action: Byte => Unit }: Unit = ()
+def for(arr: IntArray) { action: Int => Unit }: Unit = ()
+
+def print(b: Byte) = println(b)
+def print(i: Int) = println(i)
+
+def main() = {
+  val arr: IntArray = nums()
+  arr.for { x =>
+    print(x)
+  }
+}

examples/pos/issue996.effekt

@@ -4,7 +4,9 @@ type List[A] {
   Nil();
   Cons(head: A, tail: List[A])
 }
-
+namespace lists {
+  type List[A] = List[A]
+}
 
 def map[A, B](l: List[A]) { f: A => B } : List[B] =
   l match {