SI-8200 provide an identity liftable for trees

This liftable hasn't been originally included in the set of standard
liftables due to following contradiction:

1. On one hand we can have identity lifting that seems to be quite
   consistent with regular unquoting:

    q"..${List(1,2)}"        <==> q"1; 2"
    q"${List(1,2)}"          <==> q"s.c.i.List(1, 2)"
    q"..${List(q"a", q"b")}” <==> q"a; b"
    q"${List(q"a", q"b")}"   <==> q"s.c.i.List(a, b)"

   This is also consistent with how lisp unquoting works although they
   get lifting for free thanks to homoiconicity:

    // scala
    scala> val x = List(q"a", q"b); q"f($x)"
    q"f(s.c.i.List(a, b))"

    // scheme
    > (let [(x (list a b))] `(f ,x))
    '(f (list a b))

2. On the other hand lifting is an operation that converts a value into
   a code that when evaluated turns into the same value. In this sense
   Liftable[Tree] means reification of a tree into a tree that
   represents it, i.e.:

    q"${List(q"a", q"b")}"
    <==>
    q"""s.c.i.List(Ident(TermName("a")), Ident(TermName("b")))"""

   But I belive that such lifting will be very confusing for everyone
   other than a few very advanced users.

This commit introduces the first option as a default Liftable for trees.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/api/StandardLiftables.scala b/src/reflect/scala/reflect/api/StandardLiftables.scala
index af11de46ce..66ac62cc9e 100644
--- a/src/reflect/scala/reflect/api/StandardLiftables.scala
+++ b/src/reflect/scala/reflect/api/StandardLiftables.scala
@@ -27,6 +27,7 @@ trait StandardLiftables { self: Universe =>
       callScala(stdnme.Symbol)(Literal(Constant(v.name)) :: Nil)
     }
 
+    implicit def liftTree[T <: Tree]: Liftable[T]              = Liftable { identity }
     implicit def liftName[T <: Name]: Liftable[T]              = Liftable { name => Ident(name) }
     implicit def liftExpr[T <: Expr[_]]: Liftable[T]           = Liftable { expr => expr.tree }
     implicit def liftType[T <: Type]: Liftable[T]              = Liftable { tpe => TypeTree(tpe) }