Simplify type bounds.

I started out looking to limit the noise from empty type
bounds, i.e. the endless repetition of

  class A[T >: _root_.scala.Nothing <: _root_.scala.Any]

This led me to be reminded of all the unnecessary and
in fact damaging overreaches which are performed during parsing.
Why should a type parameter for which no bounds are
specified be immediately encoded with this giant tree:

  TypeBounds(
    Select(Select(Ident(nme.ROOTPKG), tpnme.scala_), tpnme.Nothing),
    Select(Select(Ident(nme.ROOTPKG), tpnme.scala_), tpnme.Any)
  )

...which must then be manually recognized as empty type bounds?
Truly, this is madness.

 - It deftly eliminates the possibility of recognizing
   whether the user wrote "class A[T]" or "class A[T >: Nothing]"
   or "class A[T <: Any]" or specified both bounds. The fact
   that these work out the same internally does not imply the
   information should be exterminated even before parsing completes.
 - It burdens everyone who must recognize type bounds trees,
   such as this author
 - It is far less efficient than the obvious encoding
 - It offers literally no advantage whatsoever

Encode empty type bounds as

  TypeBounds(EmptyTree, EmptyTree)

What could be simpler.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index a7b68ee6f8..8cf47b500d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -5149,8 +5149,8 @@ trait Typers extends Adaptations with Tags {
       }
 
       def typedTypeBoundsTree(tree: TypeBoundsTree) = {
-        val lo1 = typedType(tree.lo, mode)
-        val hi1 = typedType(tree.hi, mode)
+        val lo1 = if (tree.lo.isEmpty) TypeTree(NothingTpe) else typedType(tree.lo, mode)
+        val hi1 = if (tree.hi.isEmpty) TypeTree(AnyTpe) else typedType(tree.hi, mode)
         treeCopy.TypeBoundsTree(tree, lo1, hi1) setType TypeBounds(lo1.tpe, hi1.tpe)
       }