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, 5 insertions, 0 deletions

diff --git a/bincompat-backward.whitelist.conf b/bincompat-backward.whitelist.conf
index ae31c286f3..f9760b5c3f 100644
--- a/bincompat-backward.whitelist.conf
+++ b/bincompat-backward.whitelist.conf
@@ -102,6 +102,11 @@ filter {
         matchName="scala.collection.mutable.ArrayOps#ofDouble.unzip3"
         problemName=IncompatibleMethTypeProblem
     },
+    // see SI-8200
+    {
+        matchName="scala.reflect.api.StandardLiftables#StandardLiftableInstances.liftTree"
+        problemName=MissingMethodProblem
+    },
     // see SI-8331
     {
         matchName="scala.reflect.api.Internals#ReificationSupportApi#SyntacticTypeAppliedExtractor.unapply"