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, 7 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Erasure.scala b/src/compiler/scala/tools/nsc/transform/Erasure.scala
index e036035397..e31cb02033 100644
--- a/src/compiler/scala/tools/nsc/transform/Erasure.scala
+++ b/src/compiler/scala/tools/nsc/transform/Erasure.scala
@@ -1066,7 +1066,13 @@ abstract class Erasure extends AddInterfaces
                 if (isAccessible(qualSym) && !qualSym.isPackageClass && !qualSym.isPackageObjectClass) {
                   // insert cast to prevent illegal access error (see #4283)
                   // util.trace("insert erasure cast ") (*/
-                  treeCopy.Select(tree, gen.mkAttributedCast(qual, qual.tpe.widen), name) //)
+                  if (qualSym.isDerivedValueClass) {
+                    assert(!owner.isJavaDefined, owner)
+                    // Safe, as derived value can't extend Java classes that would be JVM inaccessible
+                    devWarning("SI-8607 Not casting qualifier ${qual} to its widened type as it is a derived value class and this cast will trip up the rest of erasure/post erasure.")
+                    tree
+                  } else
+                    treeCopy.Select(tree, gen.mkAttributedCast(qual, qual.tpe.widen), name) //)
                 } else tree
             }
           } else tree