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.

1 files changed, 6 insertions, 2 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index abc496ec0..c88342e12 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/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.