Avoid tree sharing with substituteThis

The underlying transformer has a by-name parameter for the
to provide the `to` tree, but this was strict in the layers
of API above.

Tree sharing is frowned upon in general as it leads to cross
talk when, e.g., the erasure typechecker mutates the
`tpe` field of the shared tree in different context.

Author: retronym

src/reflect/scala/reflect/api/Internals.scala

@@ -116,7 +116,7 @@ trait Internals { self: Universe =>
     /** Substitute given tree `to` for occurrences of nodes that represent
      *  `C.this`, where `C` refers to the given class `clazz`.
      */
-    def substituteThis(tree: Tree, clazz: Symbol, to: Tree): Tree
+    def substituteThis(tree: Tree, clazz: Symbol, to: => Tree): Tree
 
     /** A factory method for `ClassDef` nodes.
      */
@@ -391,7 +391,7 @@ trait Internals { self: Universe =>
         def substituteTypes(from: List[Symbol], to: List[Type]): Tree = internal.substituteTypes(tree, from, to)
 
         /** @see [[internal.substituteThis]] */
-        def substituteThis(clazz: Symbol, to: Tree): Tree = internal.substituteThis(tree, clazz, to)
+        def substituteThis(clazz: Symbol, to: => Tree): Tree = internal.substituteThis(tree, clazz, to)
       }
 
       /** Extension methods for symbols */
@@ -1143,7 +1143,7 @@ trait Internals { self: Universe =>
 
       /** @see [[InternalApi.substituteThis]] */
       @deprecated("use `internal.substituteThis` instead or import `internal.decorators._` for infix syntax", "2.11.0")
-      def substituteThis(clazz: Symbol, to: Tree): Tree = internal.substituteThis(tree, clazz, to)
+      def substituteThis(clazz: Symbol, to: => Tree): Tree = internal.substituteThis(tree, clazz, to)
     }
 
     /** Scala 2.10 compatibility enrichments for Tree. */

src/reflect/scala/reflect/internal/Internals.scala

@@ -29,7 +29,7 @@ trait Internals extends api.Internals {
     def freeTypes(tree: Tree): List[FreeTypeSymbol] = tree.freeTypes
     def substituteSymbols(tree: Tree, from: List[Symbol], to: List[Symbol]): Tree = tree.substituteSymbols(from, to)
     def substituteTypes(tree: Tree, from: List[Symbol], to: List[Type]): Tree = tree.substituteTypes(from, to)
-    def substituteThis(tree: Tree, clazz: Symbol, to: Tree): Tree = tree.substituteThis(clazz, to)
+    def substituteThis(tree: Tree, clazz: Symbol, to: => Tree): Tree = tree.substituteThis(clazz, to)
     def attachments(tree: Tree): Attachments { type Pos = Position } = tree.attachments
     def updateAttachment[T: ClassTag](tree: Tree, attachment: T): tree.type = tree.updateAttachment(attachment)
     def removeAttachment[T: ClassTag](tree: Tree): tree.type = tree.removeAttachment[T]

src/reflect/scala/reflect/internal/Trees.scala

@@ -181,7 +181,7 @@ trait Trees extends api.Trees {
     def substituteTypes(from: List[Symbol], to: List[Type]): Tree =
       new TreeTypeSubstituter(from, to)(this)
 
-    def substituteThis(clazz: Symbol, to: Tree): Tree =
+    def substituteThis(clazz: Symbol, to: => Tree): Tree =
       new ThisSubstituter(clazz, to) transform this
 
     def hasExistingSymbol = (symbol ne null) && (symbol ne NoSymbol)