Merge pull request #14666 from dotty-staging/fix-14660

Disallow private opaque type aliases

Author: odersky

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

@@ -403,7 +403,7 @@ object Denotations {
           }
         case denot1: SingleDenotation =>
           if (denot1 eq denot2) denot1
-          else if (denot1.matches(denot2)) mergeSingleDenot(denot1, denot2)
+          else if denot1.matches(denot2) then mergeSingleDenot(denot1, denot2)
           else NoDenotation
       }
 
@@ -438,8 +438,11 @@ object Denotations {
             else defn.RootClass)
 
         def isHidden(sym: Symbol) = sym.exists && !sym.isAccessibleFrom(pre)
-        val hidden1 = isHidden(sym1)
-        val hidden2 = isHidden(sym2)
+        // In typer phase filter out denotations with symbols that are not
+        // accessible. After typer, this is not possible since we cannot guarantee
+        // that the current owner is set correctly. See pos/14660.scala.
+        val hidden1 = isHidden(sym1) && ctx.isTyper
+        val hidden2 = isHidden(sym2) && ctx.isTyper
         if hidden1 && !hidden2 then denot2
         else if hidden2 && !hidden1 then denot1
         else

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

@@ -537,6 +537,7 @@ object Checking {
     checkCombination(Private, Protected)
     checkCombination(Abstract, Override)
     checkCombination(Private, Override)
+    if sym.isType && !sym.isClass then 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.

compiler/src/scala/quoted/runtime/impl/QuoteMatcher.scala

@@ -441,7 +441,7 @@ object QuoteMatcher {
   }
 
   /** Result of matching a part of an expression */
-  private opaque type Matching = Option[Tuple]
+  private type Matching = Option[Tuple]
 
   private object Matching {
 

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 be 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 +1,8 @@
+class Bar:
+  private opaque type Baz = Int // error
+
+  private object Foo:
+    opaque type O = Int // OK
+
+  val x: Baz = 1
+

tests/pos/i14660.scala

@@ -0,0 +1,6 @@
+trait Foo:
+  class Bar:
+    private[Foo] opaque type Baz = Int
+
+  def foo: Bar#Baz
+