Fix & reuse printing of dummyTreeOfType (#16750)

The main fix is the unsplice, which was making the pattern not match for
FunProto.  But also, when the tree is applied to the dummy, print the
dummy in the Apply node.  And print null as a constant literal.

Also, given AnyFunctionProto and AnyTypeConstructorProto shorter
toStrings.

Author: odersky

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -3,6 +3,7 @@ package dotc
 package printing
 
 import core._
+import Constants.*
 import Texts._
 import Types._
 import Flags._
@@ -286,14 +287,10 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case tp: ViewProto =>
         toText(tp.argType) ~ " ?=>? " ~ toText(tp.resultType)
       case tp @ FunProto(args, resultType) =>
-        val argsText = args match {
-          case dummyTreeOfType(tp) :: Nil if !(tp isRef defn.NullClass) => "null: " ~ toText(tp)
-          case _ => toTextGlobal(args, ", ")
-        }
         "[applied to ("
         ~ keywordText("using ").provided(tp.isContextualMethod)
         ~ keywordText("erased ").provided(tp.isErasedMethod)
-        ~ argsText
+        ~ argsTreeText(args)
         ~ ") returning "
         ~ toText(resultType)
         ~ "]"
@@ -309,6 +306,10 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
   protected def exprToText(tp: ExprType): Text =
     "=> " ~ toText(tp.resType)
 
+  protected def argsTreeText(args: List[untpd.Tree]): Text = args match
+    case dummyTreeOfType(tp) :: Nil if !tp.isRef(defn.NullClass) && !homogenizedView => toText(Constant(null)) ~ ": " ~ toText(tp)
+    case _                                                                           => toTextGlobal(args, ", ")
+
   protected def blockToText[T <: Untyped](block: Block[T]): Text =
     blockText(block.stats :+ block.expr)
 
@@ -442,7 +443,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
           toTextLocal(fun)
           ~ "("
           ~ Str("using ").provided(app.applyKind == ApplyKind.Using && !homogenizedView)
-          ~ toTextGlobal(args, ", ")
+          ~ argsTreeText(args)
           ~ ")"
       case tree: TypeApply =>
         typeApplyText(tree)

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

@@ -671,10 +671,12 @@ object ProtoTypes {
    *
    *    [] _
    */
-  @sharable object AnyFunctionProto extends UncachedGroundType with MatchAlways
+  @sharable object AnyFunctionProto extends UncachedGroundType with MatchAlways:
+    override def toString = "AnyFunctionProto"
 
   /** A prototype for type constructors that are followed by a type application */
-  @sharable object AnyTypeConstructorProto extends UncachedGroundType with MatchAlways
+  @sharable object AnyTypeConstructorProto extends UncachedGroundType with MatchAlways:
+    override def toString = "AnyTypeConstructorProto"
 
   extension (pt: Type)
     def isExtensionApplyProto: Boolean = pt match
@@ -946,8 +948,8 @@ object ProtoTypes {
   object dummyTreeOfType {
     def apply(tp: Type)(implicit src: SourceFile): Tree =
       untpd.Literal(Constant(null)) withTypeUnchecked tp
-    def unapply(tree: untpd.Tree): Option[Type] = tree match {
-      case Literal(Constant(null)) => Some(tree.typeOpt)
+    def unapply(tree: untpd.Tree): Option[Type] = untpd.unsplice(tree) match {
+      case tree @ Literal(Constant(null)) => Some(tree.typeOpt)
       case _ => None
     }
   }