Fix problem in decomposeProtoFunction

Need to strip again after dealiasing

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1190,6 +1190,10 @@ object Types {
     /** Strip TypeVars and Annotation and CapturingType wrappers */
     def stripped(using Context): Type = this
 
+    def strippedDealias(using Context): Type =
+      val tp1 = stripped.dealias
+      if tp1 ne this then tp1.strippedDealias else this
+
     def rewrapAnnots(tp: Type)(using Context): Type = tp.stripTypeVar match {
       case AnnotatedType(tp1, annot) => AnnotatedType(rewrapAnnots(tp1), annot)
       case _ => this

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

@@ -1130,7 +1130,7 @@ class Typer extends Namer
         case _ => mapOver(t)
     }
 
-    val pt1 = pt.stripped.dealias
+    val pt1 = pt.strippedDealias
     if (pt1 ne pt1.dropDependentRefinement)
        && defn.isContextFunctionType(pt1.nonPrivateMember(nme.apply).info.finalResultType)
     then

tests/pos-custom-args/captures/boxmap-paper.scala

@@ -5,7 +5,7 @@ type Cell[+T] = [K] => (T ==> K) => K
 def cell[T](x: T): Cell[T] =
   [K] => (k: T ==> K) => k(x)
 
-def get[T](c: Cell[T]): T = c[T](identity[T]) // TODO: drop [T]
+def get[T](c: Cell[T]): T = c[T](identity)
 
 def map[A, B](c: Cell[A])(f: A ==> B): Cell[B]
   = c[Cell[B]]((x: A) => cell(f(x)))
@@ -32,7 +32,7 @@ def test(io: {*} IO) =
 
   val r = lazyMap[{io} () => Int, Unit](c)(f => g(f))
   val r2 = lazyMap[{io} () => Int, Unit](c)(g)
-  //  val r3 = lazyMap(c)(g)  not yet
+  // val r3 = lazyMap(c)(g)
   val _ = r()
   val _ = r2()
   // val _ = r3()