SI-6187 Make partial functions re-typable

 - `New(tpe)` doesn't survive a `resetAttrs` / typecheck; use
   a name instead.
 - Abandon the tree attachment that passed the default
   case from `typer` to `patmat`; this tree eluded the
   attribute reset performed in the macro. Instead, add it
   to the match. Apart from making the tree re-typable, it
   also exposes the true code structure to macros, which
   is important if they need to perform other code
   transformations.
 - Install original trees on the declared types of the
   parameters of the `applyOrElse` method to ensure that
   references to them within the method pick up the correct
   type parameter skolems upon retypechecking.
 - Propagate `TypeTree#original` through `copyAttrs`,
   which is called during tree duplication / `TreeCopiers`.
   Without this, the original trees that we installed were
   not visible anymore during `ResetAttrs`.

We are not able to reify partial functions yet -- the
particular sticking point is reification of the parentage
which is only available in the `ClassInfoType`.

1 files changed, 18 insertions, 0 deletions

diff --git a/test/files/run/t6187.scala b/test/files/run/t6187.scala
new file mode 100644
index 0000000000..ae642917e7
--- /dev/null
+++ b/test/files/run/t6187.scala
@@ -0,0 +1,18 @@
+import scala.tools.partest.ReplTest
+
+object Test extends ReplTest {
+  override def code = """
+import language.experimental.macros, reflect.macros.Context
+def macroImpl[T: c.WeakTypeTag](c: Context)(t: c.Expr[T]): c.Expr[List[T]] = {
+  val r = c.universe.reify { List(t.splice) }
+  c.Expr[List[T]]( c.resetLocalAttrs(r.tree) )
+}
+def demo[T](t: T): List[T] = macro macroImpl[T]
+def m[T](t: T): List[List[T]] =
+  demo( List((t,true)) collect { case (x,true) => x } )
+m(List(1))
+// Showing we haven't added unreachable warnings
+List(1) collect { case x => x }
+List("") collect { case x => x }
+  """.trim
+}