Only include accessible base classes in orDominator (#16477)

By joining union types we could previously uncover inaccessible base
classes that could lead to access errors at runtime. We now filter out
such classes when computing the orDominator.

Fixes #16474

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

@@ -209,7 +209,7 @@ object TypeOps:
 
   /** Approximate union type by intersection of its dominators.
    *  That is, replace a union type Tn | ... | Tn
-   *  by the smallest intersection type of base-class instances of T1,...,Tn.
+   *  by the smallest intersection type of accessible base-class instances of T1,...,Tn.
    *  Example: Given
    *
    *      trait C[+T]
@@ -370,8 +370,14 @@ object TypeOps:
         }
       }
 
+      def isAccessible(cls: ClassSymbol) =
+        if cls.isOneOf(AccessFlags) || cls.privateWithin.exists then
+          cls.isAccessibleFrom(tp.baseType(cls).normalizedPrefix)
+        else true
+
       // Step 3: Intersect base classes of both sides
-      val commonBaseClasses = orBaseClasses(tp)
+      val commonBaseClasses = orBaseClasses(tp).filterConserve(isAccessible)
+
       val doms = dominators(commonBaseClasses, Nil)
       def baseTp(cls: ClassSymbol): Type =
         tp.baseType(cls).mapReduceOr(identity)(mergeRefinedOrApplied)

tests/pos/i16474.scala

@@ -0,0 +1,13 @@
+object o:
+
+  private[o] trait A
+  trait B
+  class C extends A, B
+  class D extends A, B
+
+def test =
+  def f[T](x: T): T => T = identity
+  val g = f(if ??? then o.C() else o.D())
+  g(new o.B{})
+
+

tests/run/i16474/BaseProvider.java

@@ -0,0 +1,6 @@
+// BaseProvider.java
+package repro.impl;
+
+import repro.*;
+abstract class BaseProvider{}
+

tests/run/i16474/Case1.java

@@ -0,0 +1,9 @@
+// Case1.java
+package repro;
+
+import repro.impl.*;
+
+public class Case1 extends Case1Provider implements Encrypter {
+  public Case1(){}
+}
+

tests/run/i16474/Case1Provider.java

@@ -0,0 +1,5 @@
+// Case1Provider.java
+package repro.impl;
+
+public abstract class Case1Provider extends BaseProvider {}
+

tests/run/i16474/Case2.java

@@ -0,0 +1,8 @@
+// Case2.java
+package repro;
+
+import repro.impl.*;
+
+public class Case2 extends Case2Provider implements Encrypter {
+  public Case2(){}
+}

tests/run/i16474/Case2Provider.java

@@ -0,0 +1,5 @@
+// Case2Provider.java
+package repro.impl;
+
+public abstract class Case2Provider extends BaseProvider {}
+

tests/run/i16474/Encrypter.java

@@ -0,0 +1,4 @@
+// Encrypter.java
+package repro;
+public interface Encrypter{}
+

tests/run/i16474/test.scala

@@ -0,0 +1,15 @@
+// scalajs: --skip
+// test.scala
+import repro.*
+import scala.util.Try
+
+def get(arg: Any): Try[Encrypter] = Try {
+  val x: Any = 1
+  arg match
+    case 1 => new Case1()
+    case 2 => new Case2()
+    case _ => throw new RuntimeException(s"Unsupported got $arg")
+}
+
+@main def Test =
+    val result = get(null)