for null outer pointer, NPE via throw null

As per the JVM spec, the athrow instruction throws a NullPointerException upon finding null on stack top.
This commit takes advantage of that feature, to emit more compact code.

The constructor of an inner class that receives an outer value first checks that value for nullness, depending on which NPE may be thrown.
The code now emitted to achieve the above looks like:

    0: aload_1
    1: ifnonnull     6
    4: aconst_null
    5: athrow
    6: ...

1 files changed, 2 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Constructors.scala b/src/compiler/scala/tools/nsc/transform/Constructors.scala
index 06367009b6..fe8bad3e3a 100644
--- a/src/compiler/scala/tools/nsc/transform/Constructors.scala
+++ b/src/compiler/scala/tools/nsc/transform/Constructors.scala
@@ -164,7 +164,8 @@ abstract class Constructors extends Transform with ast.TreeDSL {
         if (from.name != nme.OUTER ||
             from.tpe.typeSymbol.isPrimitiveValueClass) result
         else localTyper.typedPos(to.pos) {
-          IF (from OBJ_EQ NULL) THEN Throw(NewFromConstructor(NPEConstructor)) ELSE result
+          // `throw null` has the same effect as `throw new NullPointerException`, see JVM spec on instruction `athrow`
+          IF (from OBJ_EQ NULL) THEN Throw(gen.mkZero(ThrowableClass.tpe)) ELSE result
         }
       }