Reflection exceptions using structural types with Java classes

[rjmac]

Here is a case where the compiler accepts a structural type ascription and generates code which throws at runtime. It happens on at least 2.12.8 and 2.13.0:

package failure.otherpackage;

public class JavaClass {
    protected void foo() {
    }
}

package failure.somepackage

import scala.language.reflectiveCalls
import failure.otherpackage._

object Main extends App {
  val j : { def foo(): Unit } = new JavaClass  // accepted by the compiler
  j.foo() // throws an exception at runtime
}

This is probably related to #5352, which is ancient and closed. The associated fix (bedb33fd7cb) appears to have regressed. It seems the compiler treats all "non-private" access modifies as "public" when deciding whether a type matches a structural description.