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, 6 insertions, 4 deletions

diff --git a/test/files/scalacheck/quasiquotes/ErrorProps.scala b/test/files/scalacheck/quasiquotes/ErrorProps.scala
index 3d9b27de77..b07d3d90d5 100644
--- a/test/files/scalacheck/quasiquotes/ErrorProps.scala
+++ b/test/files/scalacheck/quasiquotes/ErrorProps.scala
@@ -9,9 +9,10 @@ object ErrorProps extends QuasiquoteProperties("errors") {
     """)
 
   property("can't unquote with given rank") = fails(
-    "Can't unquote List[reflect.runtime.universe.Ident], consider using ..",
+    "Can't unquote List[StringBuilder], consider using .. or providing an implicit instance of Liftable[List[StringBuilder]]",
     """
-      val xs = List(q"x", q"x")
+      import java.lang.StringBuilder
+      val xs: List[StringBuilder] = Nil
       q"$xs"
     """)
 
@@ -71,9 +72,10 @@ object ErrorProps extends QuasiquoteProperties("errors") {
     """)
 
   property("use ... rank or provide liftable") = fails(
-    "Can't unquote List[List[reflect.runtime.universe.Ident]], consider using ...",
+    "Can't unquote List[List[StringBuilder]], consider using ... or providing an implicit instance of Liftable[List[List[StringBuilder]]]",
     """
-      val xs = List(List(q"x", q"x"))
+      import java.lang.StringBuilder
+      val xs: List[List[StringBuilder]] = Nil
       q"$xs"
     """)