Avoid awkward special cases when type mapping AnnotatedTypes

A TypeMap previously mapped the annotation only if it returned a different
result for the parent type. There is no good reason for this behavior. We
now map always, but allow the mapping operation to be defined in the annotation.

The change uncovers a bug illustrated in annotDepMethType.scala, which is fixed
in the next commit.

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -113,11 +113,11 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
     case _ => 0
   }
 
-  /** The (last) list of arguments of an application */
-  def arguments(tree: Tree): List[Tree] = unsplice(tree) match {
-    case Apply(_, args) => args
-    case TypeApply(fn, _) => arguments(fn)
-    case Block(_, expr) => arguments(expr)
+  /** All term arguments of an application in a single flattened list */
+  def allArguments(tree: Tree): List[Tree] = unsplice(tree) match {
+    case Apply(fn, args) => allArguments(fn) ::: args
+    case TypeApply(fn, _) => allArguments(fn)
+    case Block(_, expr) => allArguments(expr)
     case _ => Nil
   }
 

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

@@ -9,6 +9,7 @@ import scala.util.Try
 import util.Spans.Span
 import printing.{Showable, Printer}
 import printing.Texts.Text
+import annotation.internal.sharable
 
 object Annotations {
 
@@ -28,7 +29,8 @@ object Annotations {
     def derivedAnnotation(tree: Tree)(using Context): Annotation =
       if (tree eq this.tree) this else Annotation(tree)
 
-    def arguments(using Context): List[Tree] = ast.tpd.arguments(tree)
+    /** All arguments to this annotation in a single flat list */
+    def arguments(using Context): List[Tree] = ast.tpd.allArguments(tree)
 
     def argument(i: Int)(using Context): Option[Tree] = {
       val args = arguments
@@ -46,6 +48,25 @@ object Annotations {
     /** The tree evaluation has finished. */
     def isEvaluated: Boolean = true
 
+    /** Normally, map type map over all tree nodes of this annotation, but can
+     *  be overridden. Returns EmptyAnnotation if type type map produces a range
+     *  type, since ranges cannot be types of trees.
+     */
+    def mapWith(tm: TypeMap)(using Context) =
+      val args = arguments
+      if args.isEmpty then this
+      else
+        val findDiff = new TreeAccumulator[Type]:
+          def apply(x: Type, tree: Tree)(using Context): Type =
+            if tm.isRange(x) then x
+            else
+              val tp1 = tm(tree.tpe)
+              foldOver(if tp1 =:= tree.tpe then x else tp1, tree)
+        val diff = findDiff(NoType, args)
+        if tm.isRange(diff) then EmptyAnnotation
+        else if diff.exists then derivedAnnotation(tm.mapOver(tree))
+        else this
+
     /** A string representation of the annotation. Overridden in BodyAnnotation.
      */
     def toText(printer: Printer): Text = printer.annotText(this)
@@ -200,6 +221,8 @@ object Annotations {
       apply(defn.SourceFileAnnot, Literal(Constant(path)))
   }
 
+  @sharable val EmptyAnnotation = Annotation(EmptyTree)
+
   def ThrowsAnnotation(cls: ClassSymbol)(using Context): Annotation = {
     val tref = cls.typeRef
     Annotation(defn.ThrowsAnnot.typeRef.appliedTo(tref), Ident(tref))

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

@@ -162,9 +162,13 @@ object TypeOps:
         // with Nulls (which have no base classes). Under -Yexplicit-nulls, we take
         // corrective steps, so no widening is wanted.
         simplify(l, theMap) | simplify(r, theMap)
-      case AnnotatedType(parent, annot)
-      if annot.symbol == defn.UncheckedVarianceAnnot && !ctx.mode.is(Mode.Type) && !theMap.isInstanceOf[SimplifyKeepUnchecked] =>
-        simplify(parent, theMap)
+      case tp @ AnnotatedType(parent, annot) =>
+        val parent1 = simplify(parent, theMap)
+        if annot.symbol == defn.UncheckedVarianceAnnot
+            && !ctx.mode.is(Mode.Type)
+            && !theMap.isInstanceOf[SimplifyKeepUnchecked]
+        then parent1
+        else tp.derivedAnnotatedType(parent1, annot)
       case _: MatchType =>
         val normed = tp.tryNormalize
         if (normed.exists) normed else mapOver

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

@@ -5380,6 +5380,8 @@ object Types {
       variance = saved
       derivedLambdaType(tp)(ptypes1, this(restpe))
 
+    def isRange(tp: Type): Boolean = tp.isInstanceOf[Range]
+
     /** Map this function over given type */
     def mapOver(tp: Type): Type = {
       record(s"TypeMap mapOver ${getClass}")
@@ -5423,8 +5425,9 @@ object Types {
 
         case tp @ AnnotatedType(underlying, annot) =>
           val underlying1 = this(underlying)
-          if (underlying1 eq underlying) tp
-          else derivedAnnotatedType(tp, underlying1, mapOver(annot))
+          val annot1 = annot.mapWith(this)
+          if annot1 eq EmptyAnnotation then underlying1
+          else derivedAnnotatedType(tp, underlying1, annot1)
 
         case _: ThisType
           | _: BoundType
@@ -5496,9 +5499,6 @@ object Types {
       else newScopeWith(elems1: _*)
     }
 
-    def mapOver(annot: Annotation): Annotation =
-      annot.derivedAnnotation(mapOver(annot.tree))
-
     def mapOver(tree: Tree): Tree = treeTypeMap(tree)
 
     /** Can be overridden. By default, only the prefix is mapped. */
@@ -5545,8 +5545,6 @@ object Types {
 
     protected def emptyRange = range(defn.NothingType, defn.AnyType)
 
-    protected def isRange(tp: Type): Boolean = tp.isInstanceOf[Range]
-
     protected def lower(tp: Type): Type = tp match {
       case tp: Range => tp.lo
       case _ => tp