Make sure the span of EmptyTree and EmptyValDef are never set

Author: nicolasstucki

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

@@ -113,9 +113,6 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
 
   /** Clone this node but assign it a fresh id which marks it as a node in `file`. */
   def cloneIn(src: SourceFile): this.type = {
-    assert(this ne tpd.EmptyTree)
-    assert(this ne tpd.EmptyValDef)
-    assert(this ne tpd.ContextualEmptyTree)
     val newpd: this.type = clone.asInstanceOf[this.type]
     newpd.uniqueId = src.nextId
     newpd

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

@@ -819,13 +819,15 @@ object Trees {
 
   class EmptyTree[T >: Untyped] extends Thicket(Nil)(NoSource) {
     // assert(uniqueId != 1492)
+    override def withSpan(span: Span) = throw new AssertionError("Cannot change span of EmptyTree")
   }
 
   class EmptyValDef[T >: Untyped] extends ValDef[T](
     nme.WILDCARD, genericEmptyTree[T], genericEmptyTree[T])(NoSource) with WithoutTypeOrPos[T] {
     myTpe = NoType.asInstanceOf[T]
-    override def isEmpty: Boolean = true
     setMods(untpd.Modifiers(PrivateLocal))
+    override def isEmpty: Boolean = true
+    override def withSpan(span: Span) = throw new AssertionError("Cannot change span of EmptyValDef")
   }
 
   @sharable val theEmptyTree: EmptyTree[Type] = new EmptyTree[Type]

compiler/src/dotty/tools/dotc/transform/Staging.scala

@@ -448,8 +448,8 @@ class Staging extends MacroTransformWithImplicits {
       else if (enclosingInlineds.nonEmpty) { // level 0 in an inlined call
         val spliceCtx = ctx.outer // drop the last `inlineContext`
         val pos: SourcePosition = spliceCtx.source.atSpan(enclosingInlineds.head.span)
-        val evaluatedSplice = Splicer.splice(splice.qualifier, pos, macroClassLoader)(spliceCtx).withSpan(splice.span)
-        if (ctx.reporter.hasErrors) splice else transform(evaluatedSplice)
+        val evaluatedSplice = Splicer.splice(splice.qualifier, pos, macroClassLoader)(spliceCtx)
+        if (ctx.reporter.hasErrors) splice else transform(evaluatedSplice.withSpan(splice.span))
       }
       else if (!ctx.owner.isInlineMethod) { // level 0 outside an inline method
         ctx.error(i"splice outside quotes or inline method", splice.sourcePos)