Backport "Add clause for protected visibility from package objects" (#18148)

Backports #18134
Fixes #18124

Author: Kordyjan

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/SymDenotations.scala

@@ -907,10 +907,13 @@ object SymDenotations {
           false
         val cls = owner.enclosingSubClass
         if !cls.exists then
-          val encl = if ctx.owner.isConstructor then ctx.owner.enclosingClass.owner.enclosingClass else ctx.owner.enclosingClass
-          fail(i"""
-               | Access to protected $this not permitted because enclosing ${encl.showLocated}
-               | is not a subclass of ${owner.showLocated} where target is defined""")
+          if pre.termSymbol.isPackageObject && accessWithin(pre.termSymbol.owner) then
+            true
+          else
+            val encl = if ctx.owner.isConstructor then ctx.owner.enclosingClass.owner.enclosingClass else ctx.owner.enclosingClass
+            fail(i"""
+                 | Access to protected $this not permitted because enclosing ${encl.showLocated}
+                 | is not a subclass of ${owner.showLocated} where target is defined""")
         else if isType || pre.derivesFrom(cls) || isConstructor || owner.is(ModuleClass) then
           // allow accesses to types from arbitrary subclasses fixes #4737
           // don't perform this check for static members

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