Merge pull request #5284 from milessabin/topic/si-7046

SI-7046 reflection doesn't see all knownDirectSubclasses

This appears to do the right thing in the most typical scenarios in which `knownDirectSubclasses` would be used. The missing 5% is that subclasses defined in local scopes might not be seen by `knownDirectSubclasses` (see `Local` and `Riddle` in the test below). In mitigation, though, it is almost certain that a local subclass would represent an error in any scenario where `knownDirectSubclasses` might be used.

Errors for such situations are reported by recording (via a symbol attachment) that `knownDirectSubclasses` has been called and reporting an error if any additional children are added subsequently.

Despite these limitations and caveats, I believe that this represents a huge improvement over the status quo, and would eliminate 100% of the failures that I've seen in practice with people using shapeless for type class derivation.

4 files changed, 67 insertions, 0 deletions

diff --git a/test/files/run/t7046-1/Macros_1.scala b/test/files/run/t7046-1/Macros_1.scala
new file mode 100644
index 0000000000..2a5bf82f62
--- /dev/null
+++ b/test/files/run/t7046-1/Macros_1.scala
@@ -0,0 +1,15 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.blackbox.Context
+
+object Macros {
+  def impl[T](c: Context)(implicit ttag: c.WeakTypeTag[T]): c.Expr[List[String]] = {
+    import c.universe._;
+    val ttpe = ttag.tpe
+    val tsym = ttpe.typeSymbol.asClass
+    val subclasses = tsym.knownDirectSubclasses.toList.map(_.name.toString)
+
+    c.Expr[List[String]](q"$subclasses")
+  }
+
+  def knownDirectSubclasses[T]: List[String] = macro impl[T]
+}
diff --git a/test/files/run/t7046-1/Test_2.scala b/test/files/run/t7046-1/Test_2.scala
new file mode 100644
index 0000000000..28459fde72
--- /dev/null
+++ b/test/files/run/t7046-1/Test_2.scala
@@ -0,0 +1,23 @@
+object Test extends App {
+  val subs = Macros.knownDirectSubclasses[Foo]
+  assert(subs == List("Wibble", "Wobble", "Bar", "Baz"))
+}
+
+sealed trait Foo
+object Foo {
+  trait Wibble extends Foo
+  case object Wobble extends Foo
+}
+
+trait Bar extends Foo
+
+object Blah {
+  type Quux = Foo
+}
+
+import Blah._
+
+trait Baz extends Quux
+
+class Boz[T](t: T)
+class Unrelated extends Boz(Test.subs)
diff --git a/test/files/run/t7046-2/Macros_1.scala b/test/files/run/t7046-2/Macros_1.scala
new file mode 100644
index 0000000000..2a5bf82f62
--- /dev/null
+++ b/test/files/run/t7046-2/Macros_1.scala
@@ -0,0 +1,15 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.blackbox.Context
+
+object Macros {
+  def impl[T](c: Context)(implicit ttag: c.WeakTypeTag[T]): c.Expr[List[String]] = {
+    import c.universe._;
+    val ttpe = ttag.tpe
+    val tsym = ttpe.typeSymbol.asClass
+    val subclasses = tsym.knownDirectSubclasses.toList.map(_.name.toString)
+
+    c.Expr[List[String]](q"$subclasses")
+  }
+
+  def knownDirectSubclasses[T]: List[String] = macro impl[T]
+}
diff --git a/test/files/run/t7046-2/Test_2.scala b/test/files/run/t7046-2/Test_2.scala
new file mode 100644
index 0000000000..79407f522f
--- /dev/null
+++ b/test/files/run/t7046-2/Test_2.scala
@@ -0,0 +1,14 @@
+object Test extends App {
+  def nested: Unit = {
+    sealed trait Foo
+    object Foo {
+      trait Bar extends Foo
+      trait Baz extends Foo
+    }
+
+    val subs = Macros.knownDirectSubclasses[Foo]
+    assert(subs == List("Bar", "Baz"))
+  }
+
+  nested
+}