Fix #2099: avoid loading a private member when recomputing a NamedType denot

ParamForwarding creates the following forwarder in B:
  private[this] def member: Int = super.member
Where the type for `super.member` is `TermRef(SubA, member)` and the
  symbol is the `val member` in `A`.
So far this is correct, but in later phases we might call `loadDenot` on
this `TermRef` which will end up calling `asMemberOf`, which before this
commit just did:
  prefix.member(name)
This is incorrect in our case because `SubA` also happens to have a
private `def member`, which means that our forwarder in B now forwards
to a private method in a superclass, this subsequently crashes in
`ExpandPrivate`.
(Note: in the bytecode, a private method cannot have the same name as an
overriden method, but this is already worked around in EnsurePrivate.)

The fix is simple: when we recompute a member, we should only look at
private members if the previous denotation was private.

Author: smarter

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1585,8 +1585,12 @@ object Types {
 
     protected def asMemberOf(prefix: Type)(implicit ctx: Context): Denotation =
       if (name.isShadowedName) prefix.nonPrivateMember(name.revertShadowed)
-      else prefix.member(name)
-
+      else {
+        val d = lastDenotation
+        // Never go from a non-private denotation to a private one
+        if (d == null || d.symbol.is(Private)) prefix.member(name)
+        else prefix.nonPrivateMember(name)
+      }
 
     /** (1) Reduce a type-ref `W # X` or `W { ... } # U`, where `W` is a wildcard type
      *  to an (unbounded) wildcard type.

tests/run/paramForwarding_separate.check

@@ -0,0 +1,6 @@
+# Fields in A:
+private final int A.member$$local
+# Fields in SubA:
+
+# Fields in B:
+

tests/run/paramForwarding_separate/A_1.scala

@@ -0,0 +1,3 @@
+class A(val member: Int)
+
+class SubA(member: Int) extends A(member)

tests/run/paramForwarding_separate/B_2.scala

@@ -0,0 +1,19 @@
+class B(member: Int) extends SubA(member)
+
+object Test {
+  def printFields(cls: Class[_]) =
+    println(cls.getDeclaredFields.map(_.toString).sorted.deep.mkString("\n"))
+
+  def main(args: Array[String]): Unit = {
+    val a = new A(10)
+    val subA = new SubA(11)
+    val b = new B(12)
+
+    println("# Fields in A:")
+    printFields(classOf[A])
+    println("# Fields in SubA:")
+    printFields(classOf[SubA])
+    println("# Fields in B:")
+    printFields(classOf[B])
+  }
+}