Better fix that handles the missing corner case

Author: odersky

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

@@ -441,6 +441,14 @@ object Contexts {
         .setScope(this.scope)
     }
 
+    /** Is this a super call context?
+     *  This is the case if we are in a primary constructor and
+     *  the outer context has as owner the owner of the enclosing class.
+     *  The enclosing class is `owner.owner`, hence `outer.owner == owner.owner.owner`.
+     */
+    def isSuperCallContext: Boolean =
+      owner.isPrimaryConstructor && outer.owner == owner.owner.owner
+
     /** The super- or this-call context with given owner and locals. */
     private def superOrThisCallContext(owner: Symbol, locals: Scope): FreshContext = {
       val classCtx = outersIterator.dropWhile(!_.isClassDefContext).next()

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

@@ -942,19 +942,16 @@ object SymDenotations {
       }
 
       /** Is protected access to target symbol permitted? */
-      def isProtectedAccessOK: Boolean =
+      def isProtectedAccessOK: Boolean = {
         val cls = owner.enclosingSubClass
         if !cls.exists then
           pre.termSymbol.isPackageObject && accessWithin(pre.termSymbol.owner)
-        else {
-          val isConstructorAccessOK =
-            isConstructor && ctx.owner.isPrimaryConstructor
-            && cls.info.parents.exists(_.classSymbol == owner)
+        else
+          val isConstructorAccessOK = isConstructor && ctx.isSuperCallContext
           // allow accesses to types from arbitrary subclasses fixes #4737
           // don't perform this check for static members
           isType || pre.derivesFrom(cls) || isConstructorAccessOK || owner.is(ModuleClass)
-        }
-      end isProtectedAccessOK
+      }
 
       if pre eq NoPrefix then true
       else if isAbsent() then false

tests/neg/i25442.check

@@ -0,0 +1,34 @@
+-- [E173] Reference Error: tests/neg/i25442/protected-constructors.scala:7:22 ------------------------------------------
+7 |class C extends B(new A(1)) // error
+  |                      ^
+  |   constructor A cannot be accessed as a member of protectedCtors.A from class C.
+  |     protected constructor A can only be accessed from class C in package protectedCtors or one of its subclasses.
+-- [E173] Reference Error: tests/neg/i25442/protected-constructors.scala:10:22 -----------------------------------------
+10 |class E extends D(new D(1)) // error
+   |                      ^
+   | constructor D cannot be accessed as a member of protectedCtors.D from class E.
+   |   protected constructor D can only be accessed from class E in package protectedCtors or one of its subclasses.
+-- Error: tests/neg/i25442/protected-constructors.scala:18:24 ----------------------------------------------------------
+18 |class J extends G(new F(1)) // error: protected primary in super args
+   |                        ^
+   |                        too many arguments for constructor F in class F: (): protectedCtors.F
+-- [E173] Reference Error: tests/neg/i25442/protected-constructors.scala:22:14 -----------------------------------------
+22 |  val k = new A(1) // error
+   |              ^
+   | constructor A cannot be accessed as a member of protectedCtors.A from class K.
+   |   protected constructor A can only be accessed from class K in package protectedCtors or one of its subclasses.
+-- [E173] Reference Error: tests/neg/i25442/protected-constructors.scala:27:28 -----------------------------------------
+27 |class N extends M(() => new A(2)) // error
+   |                            ^
+   | constructor A cannot be accessed as a member of protectedCtors.A from class N.
+   |   protected constructor A can only be accessed from class N in package protectedCtors or one of its subclasses.
+-- [E173] Reference Error: tests/neg/i25442/test.scala:7:15 ------------------------------------------------------------
+7 |  def t2 = new I(42) // error
+  |               ^
+  |               constructor I cannot be accessed as a member of I from class M.
+  |                 protected constructor I can only be accessed from class M or one of its subclasses.
+-- [E173] Reference Error: tests/neg/i25442/test.scala:8:35 ------------------------------------------------------------
+8 |  def this(x: Int) = { this(); new I(x) } // error
+  |                                   ^
+  |                             constructor I cannot be accessed as a member of I from class M.
+  |                               protected constructor I can only be accessed from class M or one of its subclasses.

tests/neg/i25442/protected-constructors.scala

@@ -6,6 +6,9 @@ class A protected (x: Int)
 class B(a: A) extends A(a.hashCode)
 class C extends B(new A(1)) // error
 
+class D protected (x: Any)
+class E extends D(new D(1)) // error
+
 // Mixed visibility constructors
 class F protected (x: Int) {
   def this() = this(0) // public secondary