Merge multiple parameter lists in resolveOverloaded

Resolve overloaded does an applicability test which can make use of only a single
type parameter list. When going deeper into several alternatives that contain a common
prefex we might uncover other type parameter lists. These need to be merged.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1903,7 +1903,7 @@ trait Applications extends Compatibility {
       def skip(tp: Type): Type = tp match {
         case tp: PolyType =>
           val rt = skip(tp.resultType)
-          if (rt.exists) tp.derivedLambdaType(resType = rt) else rt
+          if rt.exists then tp.derivedLambdaType(resType = rt).asInstanceOf[PolyType].flatten else rt
         case tp: MethodType =>
           tp.instantiate(argTypes)
         case _ =>
@@ -1974,7 +1974,7 @@ trait Applications extends Compatibility {
         None
     }
     val mapped = reverseMapping.map(_._1)
-    overload.println(i"resolve mapped: $mapped")
+    overload.println(i"resolve mapped: ${mapped.map(_.widen)}%, % with $pt")
     resolveOverloaded(mapped, pt).map(reverseMapping.toMap)
 
   /** Try to typecheck any arguments in `pt` that are function values missing a

tests/pos/i11358.scala

@@ -0,0 +1,8 @@
+object Test:
+
+  def test1: IArray[Int] = IArray(1, 2) +++ IArray(2, 3)
+  def test2: IArray[Int] = IArray(1, 2) +++ List(2, 3)
+
+  extension [A: reflect.ClassTag](arr: IArray[A])
+    def +++[B >: A: reflect.ClassTag](suffix: IArray[B]): IArray[B] = ???
+    def +++[B >: A: reflect.ClassTag](suffix: IterableOnce[B]): IArray[B] = ???