SI-8233 Fix regression in backend with boxed nulls

Regressed in SI-7015 / 1b6661b8.

We do need to "unbox" the null (ie, drop a stack from and load
a null) in general. The only time we can avoid this is if the
tree we are adapting is a `Constant(Literal(null))`.

I've added a test for both backends. Only GenICode exhibited
the problem.

1 files changed, 12 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala b/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
index 1332d01dbd..9ba38b021d 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
@@ -1007,8 +1007,15 @@ abstract class GenICode extends SubComponent  {
       }
 
       // emit conversion
-      if (generatedType != expectedType)
-        adapt(generatedType, expectedType, resCtx, tree.pos)
+      if (generatedType != expectedType) {
+        tree match {
+          case Literal(Constant(null)) if generatedType == NullReference =>
+            // literal null on the stack (as opposed to a boxed null, see SI-8233),
+            // we can bypass `adapt` which would otherwise emitt a redundant [DROP, CONSTANT(null)]
+          case _ =>
+            adapt(generatedType, expectedType, resCtx, tree.pos)
+        }
+      }
 
       resCtx
     }
@@ -1058,6 +1065,9 @@ abstract class GenICode extends SubComponent  {
         case (NothingReference, _) =>
           ctx.bb.emit(THROW(ThrowableClass))
           ctx.bb.enterIgnoreMode()
+        case (NullReference, REFERENCE(_)) =>
+          // SI-8223 we can't assume that the stack contains a `null`, it might contain a Null$
+          ctx.bb.emit(Seq(DROP(from), CONSTANT(Constant(null))))
         case _ if from isAssignabledTo to =>
           ()
         case (_, UNIT) =>