Weaken checkPos

With given coming last, some of the no-overlaps and in-order position
checks have to be weakened. But with the more generalized pathTo
implementation, non-overlapping and in-order is not really needed anymore.

Author: odersky

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

@@ -147,6 +147,11 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
       }
   }
 
+  /** A hook that can be overridden if overlap checking in `checkPos` should be
+   *  disabled for this node.
+   */
+  def disableOverlapChecks = false
+
   /** Check that all positioned items in this tree satisfy the following conditions:
    *  - Parent spans contain child spans
    *  - If item is a non-empty tree, it has a position
@@ -169,7 +174,7 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
               s"position error: position not set for $tree # ${tree.uniqueId}")
           case _ =>
         }
-        if (nonOverlapping) {
+        if (nonOverlapping && !disableOverlapChecks) {
           this match {
             case _: XMLBlock =>
               // FIXME: Trees generated by the XML parser do not satisfy `checkPos`

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

@@ -746,6 +746,10 @@ object Trees {
     assert(tpt != genericEmptyTree)
     def unforced: LazyTree = preRhs
     protected def force(x: AnyRef): Unit = preRhs = x
+
+    override def disableOverlapChecks = rawMods.is(Flags.Implied)
+      // disable order checks for implicit aliases since their given clause follows
+      // their for clause, but the two appear swapped in the DefDef.
   }
 
   class BackquotedDefDef[-T >: Untyped] private[ast] (name: TermName, tparams: List[TypeDef[T]],
@@ -783,6 +787,10 @@ object Trees {
 
     def parents: List[Tree[T]] = parentsOrDerived // overridden by DerivingTemplate
     def derived: List[untpd.Tree] = Nil           // overridden by DerivingTemplate
+
+    override def disableOverlapChecks = true
+      // disable overlaps checks since templates of instance definitions have their
+      // `given` clause come last, which means that the constructor span can contain the parent spans.
   }
 
 

tests/pos/reference/instances.scala

@@ -132,7 +132,7 @@ object Instances extends Common {
         println(the[D[Int]])
       }
       locally {
-        implied given Context for D[Int]
+        implied for D[Int] given Context
         println(the[D[Int]])
       }
     }