checkNoPrivateLeaks: Do not allow types to refer to leaky aliases

`checkNoPrivateLeaks` can force a lot of things, this lead to
hard-to-reproduce issues in unpickling because we called
`checkNoPrivateLeaks` on the type parameters of a class before anything
in the class was indexed. We fix this by making sure that
`checkNoPrivateLeaks` never transforms type symbols, only term symbols,
therefore we can unpickle type parameters without forcing too many
things. tests/neg/leak-type.scala illustrates the new restriction that
this necessitates.

3 files changed, 28 insertions, 1 deletions

diff --git a/tests/neg/leak-type.scala b/tests/neg/leak-type.scala
new file mode 100644
index 000000000..30ecab70b
--- /dev/null
+++ b/tests/neg/leak-type.scala
@@ -0,0 +1,13 @@
+trait A {
+  private type Foo = Int
+
+
+  class Inner[T <: Foo] { // error: non-private type T refers to private type Foo in its type signature
+    def get: T = ???
+  }
+}
+class B extends A {
+  def foo(x: Inner[_]): Unit = {
+    val a = x.get // error: cannot resolve reference to type B(B.this).Foo
+  }
+}
diff --git a/tests/pos/i1130.scala b/tests/pos/i1130.scala
index 8d71de5e8..c28eaa169 100644
--- a/tests/pos/i1130.scala
+++ b/tests/pos/i1130.scala
@@ -3,4 +3,6 @@ trait A {
 
   def foo: Foo = 1
 }
-class B extends A
+class B extends A {
+  foo
+}
diff --git a/tests/pos/leak-inferred.scala b/tests/pos/leak-inferred.scala
new file mode 100644
index 000000000..5d8a7e3bc
--- /dev/null
+++ b/tests/pos/leak-inferred.scala
@@ -0,0 +1,12 @@
+class A {
+  private val x = List(1,2)
+
+  val elem = x.head
+}
+
+class B extends A {
+  val a: Int = elem
+    // Without `checkNoPrivateLeaks`, we get:
+    // found:    B.this.x.scala$collection$immutable$List$$A(B.this.elem)
+    // required: Int
+}