SI-7015 Removes redundant aconst_null; pop; aconst_null creation

In an effort to adapt methods and field accesses of type Null to
other types, we were always emitting

    aconst_null
    pop
    aconst_null

The problem is we were doing that even when the JVM was in a position
to know it had null value, e.g. when the user had written a null
constant. This commit fixes that and includes a test to show that the
resulting byte code still works even without repeating ourselves and/or
repeating ourselves.

This commit also makes the scala.runtim.Null$ constructor private. It
was a sealed abstract class which prevented subclassing in Scala, but
it didn't prevent subclassing in Java. A private constructor takes care
of that hole so now the only value of type Null$ should be null.

Along the way I found some other questionable things in adapt and I've
added TODO's and issue https://issues.scala-lang.org/browse/SI-7159 to
track.

2 files changed, 60 insertions, 0 deletions

diff --git a/test/files/run/t7015.check b/test/files/run/t7015.check
new file mode 100644
index 0000000000..7651fe06b0
--- /dev/null
+++ b/test/files/run/t7015.check
@@ -0,0 +1,11 @@
+Method returns Null type: null
+Method takes non Null type: null
+call through method null
+call through bridge null
+fetch field: null
+fetch field on companion: null
+fetch local: null
+fetch array element: null
+method that takes object: null
+method that takes anyref: null
+method that takes any: null
diff --git a/test/files/run/t7015.scala b/test/files/run/t7015.scala
new file mode 100644
index 0000000000..9344ca2906
--- /dev/null
+++ b/test/files/run/t7015.scala
@@ -0,0 +1,49 @@
+object Test {
+  def main(args : Array[String]) : Unit = {
+    println(s"Method returns Null type: $f")
+    println(s"Method takes non Null type: ${g(null)}")
+
+    // pass things through the g function because it expects
+    // a string. If we haven't adapted properly then we'll
+    // get verify errors
+    val b = new B
+    println(s"call through method ${g(b.f(null))}")
+    println(s"call through bridge ${g((b: A).f(null))}")
+
+    println(s"fetch field: ${g(b.nullField)}")
+    println(s"fetch field on companion: ${g(B.nullCompanionField)}")
+
+    val x = f
+    println(s"fetch local: ${g(x)}")
+
+    val nulls = Array(f, f, f)
+    println(s"fetch array element: ${g(nulls(0))}")
+
+    println(s"method that takes object: ${q(f)}")
+    println(s"method that takes anyref: ${r(f)}")
+    println(s"method that takes any: ${s(f)}")
+  }
+
+  def f = null
+
+  def g(x: String) = x
+
+  def q(x: java.lang.Object) = x
+  def r(x: AnyRef) = x
+  def s(x: Any) = x
+}
+
+abstract class A {
+	def f(x: String): String
+}
+
+class B extends A {
+	val nullField = null
+
+	// this forces a bridge method because the return type is different
+	override def f(x: String) : Null = null
+}
+
+object B {
+	val nullCompanionField = null
+}
\ No newline at end of file