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.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/internal/StdAttachments.scala b/src/reflect/scala/reflect/internal/StdAttachments.scala
index fd8f51cfb1..fc49de1cf6 100644
--- a/src/reflect/scala/reflect/internal/StdAttachments.scala
+++ b/src/reflect/scala/reflect/internal/StdAttachments.scala
@@ -81,4 +81,10 @@ trait StdAttachments {
 
   /** An attachment carrying information between uncurry and erasure */
   case class TypeParamVarargsAttachment(val typeParamRef: Type)
+
+  /** Attached to a class symbol to indicate that its children have been observed
+    * via knownDirectSubclasses. Children added subsequently will trigger an
+    * error to indicate that the earlier observation was incomplete.
+    */
+  case object KnownDirectSubclassesCalled extends PlainAttachment
 }