Make sure the span of EmptyTree and EmptyValDef are never set

It is important not to set the span as if the sources differ the tree will be cloned
and the EmptyTree will have multiple instances which are not comparable.

Author: nicolasstucki

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)