Add neg test for checking private leaks

Author: odersky

tests/neg/export-leaks.scala

@@ -0,0 +1,22 @@
+// Check that exports do not leak private types
+object Signature {
+
+  private type T
+
+  object O1 {
+    private[Signature] def bar: T = ???
+  }
+  export O1._ // error: non-private method bar refers to private type T
+
+  object O2 {
+    private[Signature] val foo: T = ???
+  }
+  export O2._ // OK
+      // The reason this works is that private escape checking only looks at real private members,
+      // not at private[C] members. So, by itself the expansion of the export
+      //   <stable> def foo: O2.foo.type = O2.foo
+      // is legal. The underlying type of `O2.foo.type` does violate no-escape rules, but we do not
+      // check for it. Maybe we should. But then the question comes up whether we should
+      // also check all possible supertypes of a type for privacy violations. These are more
+      // general questions that are not related to exports.
+}