Make toplevel protected qualified protected

For private toplevel definitions we expand `private` to `private[p]`
where `p` is the enclosing package. This PR applies the same scheme to
protected toplevel definitions.

Rationale: #18057 fixed an issue where toplevel protected members were always accessible
because explicit package object prefixes were added after the accessibility check was done,
and would re-establish the previous members without doing an accessibility check. The fix was
done by adding package objects first, then doing he rest of the checks. But that also means
that protected toplevel objects now get checked as members of their synthetic package object
instead of as members of their package. To avoid that we make the package explicit as qalifier.

Thsi shouls also make specs2 compile again.

Author: odersky

community-build/test/scala/dotty/communitybuild/CommunityBuildTest.scala

@@ -93,11 +93,7 @@ class CommunityBuildTestC:
   @Test def sconfig = projects.sconfig.run()
   @Test def shapeless = projects.shapeless.run()
   @Test def sourcecode = projects.sourcecode.run()
-
-  // Disabled. Currently fails in FutureMatchers.scala. The call to
-  // `checkResultFailure` goes to a protected method which is not accessible.
-  // I tried to fix it, but get test failures.
-  // @Test def specs2 = projects.specs2.run()
+  @Test def specs2 = projects.specs2.run()
 
   @Test def stdLib213 = projects.stdLib213.run()
   @Test def ujson = projects.ujson.run()

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

@@ -600,6 +600,7 @@ object Flags {
   val TermParamOrAccessor: FlagSet           = Param | ParamAccessor
   val PrivateParamAccessor: FlagSet          = ParamAccessor | Private
   val PrivateOrArtifact: FlagSet             = Private | Artifact
+  val PrivateOrProtected: FlagSet            = Private | Protected
   val ClassTypeParam: FlagSet                = Private | TypeParam
   val Scala2Trait: FlagSet                   = Scala2x | Trait
   val SyntheticArtifact: FlagSet             = Synthetic | Artifact

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

@@ -223,11 +223,10 @@ class Namer { typer: Typer =>
       var flags1 = flags
       var privateWithin = privateWithinClass(tree.mods)
       val effectiveOwner = owner.skipWeakOwner
-      if (flags.is(Private) && effectiveOwner.is(Package)) {
-        // If effective owner is a package p, widen private to private[p]
-        flags1 = flags1 &~ PrivateLocal
+      if (flags.isOneOf(PrivateOrProtected) && !privateWithin.exists) && effectiveOwner.is(Package) then
+        // If effective owner is a package p, widen private to private[p] and protected to protected[p]
+        flags1 = flags1 &~ PrivateLocal // drop private but keep protected
         privateWithin = effectiveOwner
-      }
 
       val sym =
         if (prev.exists) {

tests/pos/i18124/definition.scala

@@ -0,0 +1,15 @@
+// definition.scala
+package oolong.bson:
+
+  trait BsonValue
+  protected def merge(
+      base: BsonValue,
+      patch: BsonValue,
+      arraySubvalues: Boolean = false
+  ): BsonValue = ???
+
+  private def foo: Int = 1
+
+  package inner:
+    protected[bson] def bar = 2
+

tests/pos/i18124/usage.scala

@@ -0,0 +1,8 @@
+// usage.scala
+package oolong.bson
+
+extension (bv: BsonValue)
+  def :+(other: BsonValue): BsonValue = merge(other, bv, false)
+
+val x = foo
+val y = inner.bar