SI-8607 Fix erasure for value class inheriting from private class

The fix for SI-4283 added a pre-emptive cast of the qualifier of
a selection `qual.a` to avoid later casting to the owner of `a`
if that owner might be inaccessible at the call site. This fixed
a `LinkageError`.

In the enclosed test, this cast led to the following diff in the
tree shapes (with respect to a version with a public base class).

    [erasure]
    -        new p1.C.<init>(c.$asInstanceOf[scala.this.Int]()).a();
    +        new p1.C.<init>(new p1.C.<init>(c.$asInstanceOf[scala.this.Int]()).$asInstanceOf[ErasedValueType(    class C, scala.this.Int)]().$asInstanceOf[scala.this.Int]()).a();
    [posterasure]
    -        new p1.C.<init>(c).a();
    +        new p1.C.<init>(new p1.C.<init>(c).$asInstanceOf[scala.this.Int]().$asInstanceOf[scala.this.Int]())    .a();
             ()

What we really wanted to end up with is:

    new p1.C.<init>(c).$asInstanceOf[C]().a();

The stray cast leads to the crash:

    error: should have been unboxed by erasure: new p1.C.<init>(c).$asInstanceOf[scala.this.Int]()

Rather than trying to fix this in erasure/posterasure, this commit
instead relies on the fact the owner of `a` cannot be Java defined
if `qual`s type is a derived value class. This follows from the
restrictions we place on value classes.

With this knowledge, we elide the cast altogether in this case.

1 files changed, 36 insertions, 0 deletions

diff --git a/test/files/run/t8607.scala b/test/files/run/t8607.scala
new file mode 100644
index 0000000000..1b8ef9bbd0
--- /dev/null
+++ b/test/files/run/t8607.scala
@@ -0,0 +1,36 @@
+package p1 {
+  private[p1] trait B extends Any {
+    def a: Any = ""
+  }
+ 
+  class C(val value: Int) extends AnyVal with B {
+    // def b = ""
+  }
+}
+
+object Test {
+  def main(args: Array[String]) {
+    val c = new p1.C(42)
+    c.a
+    /*
+    new p1.C.<init>(
+      c.$asInstanceOf[scala.this.Int]()
+    ).a();
+
+
+    new p1.C.<init>(
+      new p1.C.<init>(
+        c.$asInstanceOf[scala.this.Int]()
+      ).$asInstanceOf[ErasedValueType(class C, scala.this.Int)]()
+       .$asInstanceOf[scala.this.Int]()
+    ).a();
+
+    new p1.C.<init>(
+      new p1.C.<init>(c)
+        .$asInstanceOf[scala.this.Int]()
+        .$asInstanceOf[scala.this.Int]()
+      ).a();
+
+    */
+  }
+}