Eliminate TypeTrees with null original.

This is a retry of #2801 after figuring out the range position
error. Should there be anyone out there who compiles with -Xdev,
know that this commit eliminates the 1406 errors one presently
incurs compiling src/library.

A val declared in source code receives only one tree from the
parser, but two are needed - one for the field and one for the
getter. I discovered long ago that if the val had an existential
type, this was creating issues with incompatible existentials
between the field and the getter. However the remedy for that
did not take into account the whole of the wide range of super
subtle issues which accompany tree duplication.

In particular, the duplicated tree must be given not only a
fresh TypeTree(), but that TypeTree cannot share the same
original without running afoul of range position invariants.
That's because typedTypeTree resurrects the original tree with
whatever position it has - so the "original" needs to be a
duplicate of the original with a focused position.

Should the call to TypeTree.duplicate also duplicate the original?
I think so, but I bequeath this question to others.

This commit also eliminated some duplicate error messages, because
duplicate suppression depends on the errors having the same position.

See c478eb770d, 7a6fa80937 for previous related work.

1 files changed, 9 insertions, 8 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
index 3a5845c8ca..263b5ad784 100644
--- a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
@@ -397,6 +397,12 @@ trait MethodSynthesis {
         if (mods.isDeferred) basisSym
         else basisSym.getter(enclClass)
       )
+      // Range position errors ensue if we don't duplicate this in some
+      // circumstances (at least: concrete vals with existential types.)
+      private def tptOriginal = (
+        if (mods.isDeferred) tree.tpt                       // keep type tree of original abstract field
+        else tree.tpt.duplicate setPos tree.tpt.pos.focus   // focused position of original tpt
+      )
 
       override def derivedTree: DefDef = {
         // For existentials, don't specify a type for the getter, even one derived
@@ -407,16 +413,11 @@ trait MethodSynthesis {
         // starts compiling (instead of failing like it's supposed to) because the typer
         // expects to be able to identify escaping locals in typedDefDef, and fails to
         // spot that brand of them. In other words it's an artifact of the implementation.
-        val tpt = derivedSym.tpe.finalResultType match {
+        val tpt = atPos(derivedSym.pos.focus)(derivedSym.tpe.finalResultType match {
           case ExistentialType(_, _)  => TypeTree()
           case _ if mods.isDeferred   => TypeTree()
           case tp                     => TypeTree(tp)
-        }
-        tpt setPos derivedSym.pos.focus
-        // keep type tree of original abstract field
-        if (mods.isDeferred)
-          tpt setOriginal tree.tpt
-
+        })
         // TODO - reconcile this with the DefDef creator in Trees (which
         //   at this writing presented no way to pass a tree in for tpt.)
         atPos(derivedSym.pos) {
@@ -425,7 +426,7 @@ trait MethodSynthesis {
             derivedSym.name.toTermName,
             Nil,
             Nil,
-            tpt,
+            tpt setOriginal tptOriginal,
             if (mods.isDeferred) EmptyTree else fieldSelection
           ) setSymbol derivedSym
         }