Add missing criterion to subtype check

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

@@ -309,6 +309,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 thirdTryNamed(tp2)
               else
                 (  (tp1.name eq tp2.name)
+                && !sym1.is(Private)
                 && tp2.isPrefixDependentMemberRef
                 && isSubPrefix(tp1.prefix, tp2.prefix)
                 && tp1.signature == tp2.signature

tests/neg/i16850.check

@@ -0,0 +1,10 @@
+-- [E007] Type Mismatch Error: tests/neg/i16850.scala:7:33 -------------------------------------------------------------
+7 |  def add(elm: Y): Unit = list = elm :: list // error
+  |                                 ^^^
+  |                                 Found:    (elm : Y)
+  |                                 Required: Class.this.Y²
+  |
+  |                                 where:    Y  is a type in class Class
+  |                                           Y² is a type in trait Trait
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i16850.scala

@@ -0,0 +1,10 @@
+
+trait Trait :
+  type Y
+  var list: List[Y] = Nil
+
+class Class[Y] extends Trait :
+  def add(elm: Y): Unit = list = elm :: list // error
+
+object Object extends Class[Int] :
+  add(42)