Disallow private opaque type aliases

Fixes scala#14660

Author: odersky

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -537,6 +537,7 @@ object Checking {
     checkCombination(Private, Protected)
     checkCombination(Abstract, Override)
     checkCombination(Private, Override)
+    checkCombination(Private, Opaque)
     checkCombination(Lazy, Inline)
     // The issue with `erased inline` is that the erased semantics get lost
     // as the code is inlined and the reference is removed before the erased usage check.

docs/_docs/reference/other-new-features/opaques-details.md

@@ -51,6 +51,8 @@ object o:
 def id(x: o.T): o.T = x
 ```
 
+Opaque type aliases cannot be `private` and cannot overridden in subclasses.
+
 ## Type Parameters of Opaque Types
 
 Opaque type aliases can have a single type parameter list. The following aliases

tests/neg/i14660.scala

@@ -0,0 +1,2 @@
+class Bar:
+  private opaque type Baz = Int // error