Allow override protected[C] in companion

When extending C in its companion, allow
override protected[C] where C denotes the
enclosing companion module of C.

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

@@ -414,13 +414,13 @@ object RefChecks {
       val ob = other.accessBoundary(member.owner)
       val mb = member.accessBoundary(member.owner)
       def isOverrideAccessOK =
-           (member.flags & AccessFlags).isEmpty
-           && !member.privateWithin.exists // member is public, or
-        || (!other.is(Protected) || member.is(Protected))
-              // if o is protected, so is m, and
-           && (ob.isContainedIn(mb) || other.isAllOf(JavaProtected))
-             // m relaxes o's access boundary,
-             // or o is Java defined and protected (see #3946)
+        val memberIsPublic = (member.flags & AccessFlags).isEmpty && !member.privateWithin.exists
+        def protectedOK = !other.is(Protected) || member.is(Protected)        // if o is protected, so is m
+        def companionBoundaryOK = ob.isClass && mb.is(Module) && (ob.companionModule eq mb.companionModule)
+        def accessBoundaryOK = ob.isContainedIn(mb) || companionBoundaryOK    // m relaxes o's access boundary,
+        def otherIsJavaProtected = other.isAllOf(JavaProtected)               // or o is Java defined and protected (see #3946)
+        memberIsPublic || protectedOK && (accessBoundaryOK || otherIsJavaProtected)
+      end isOverrideAccessOK
       if !member.hasTargetName(other.targetName) then
         overrideTargetNameError()
       else if (!isOverrideAccessOK)

tests/pos/t12494.scala

@@ -0,0 +1,29 @@
+
+trait Base {
+  protected[Base] def f: Int
+}
+object Base {
+  class Child extends Base {
+    protected[Base] def f: Int = 42   // ok
+    def test = f
+  }
+}
+
+/*
+object X {
+  // restriction in Scala 2 for local companions
+  // restriction in Scala 3 under -from-tasty
+  def m: Int = {
+    trait C {
+      protected[C] def f: Int
+    }
+    object C {
+      class C2 extends C {
+        protected[C] def f: Int = 42  // ok
+        def test = f
+      }
+    }
+    C.C2().test
+  }
+}
+*/