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.

4 files changed, 49 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) =>
diff --git a/test/files/run/t8233-bcode.flags b/test/files/run/t8233-bcode.flags
new file mode 100644
index 0000000000..c30091d3de
--- /dev/null
+++ b/test/files/run/t8233-bcode.flags
@@ -0,0 +1 @@
+-Ybackend:GenBCode
diff --git a/test/files/run/t8233-bcode.scala b/test/files/run/t8233-bcode.scala
new file mode 100644
index 0000000000..fae1c2b702
--- /dev/null
+++ b/test/files/run/t8233-bcode.scala
@@ -0,0 +1,18 @@
+object Test {
+  def bar(s: String) = s;
+  val o: Option[Null] = None
+  def nullReference {
+    val a: Null = o.get
+    bar(a) // Was: VerifyError under GenICode
+  }
+
+  def literal {
+    val a: Null = null
+    bar(a)
+  }
+
+  def main(args: Array[String]) = {
+    try { nullReference } catch { case _: NoSuchElementException => }
+    literal
+  }
+}
diff --git a/test/files/run/t8233.scala b/test/files/run/t8233.scala
new file mode 100644
index 0000000000..fae1c2b702
--- /dev/null
+++ b/test/files/run/t8233.scala
@@ -0,0 +1,18 @@
+object Test {
+  def bar(s: String) = s;
+  val o: Option[Null] = None
+  def nullReference {
+    val a: Null = o.get
+    bar(a) // Was: VerifyError under GenICode
+  }
+
+  def literal {
+    val a: Null = null
+    bar(a)
+  }
+
+  def main(args: Array[String]) = {
+    try { nullReference } catch { case _: NoSuchElementException => }
+    literal
+  }
+}