Infer dotted package name for visibility

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -375,12 +375,16 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     val ctxs = ctx.outersIterator
     while !done && ctxs.hasNext do
       val cur = ctxs.next()
-      if cur.owner eq sym then
-        if importer != null && sym.is(Package) then
-          done = true // an enclosing package might be imported because p.q not visible as q
+      if cur.owner.userSymbol eq sym then
+        if sym.is(Package) then
+          if cur.isSimplePackage then
+            precedence = Definition
+            importer = null
+          // else package p.q is not available as a simple name but it might be imported
+          end if
+          done = true
         else
-          addCached(cachePoint, Definition)
-          return // found enclosing definition
+          return // found enclosing definition, don't register the reference
       else if isLocal then
         if cur.owner eq sym.owner then
           done = true // for local def, just checking that it is not enclosing
@@ -966,6 +970,23 @@ object CheckUnused:
     inline def userSymbol: Symbol=
       if sym.denot.is(ModuleClass) then sym.denot.companionModule else sym
 
+  extension (cur: Context)
+    def isSimplePackage(using Context): Boolean = cur.owner.is(Package) && {
+      val location = cur.owner.showFullName
+      val enclosing = cur.outersIterator
+        .takeWhile(c => !c.owner.isEmptyPackage && !(c.isImportContext && c.importInfo.nn.isRootImport))
+        .find(c => c.owner.userSymbol ne cur.owner.userSymbol)
+      enclosing match
+      case Some(outer) =>
+        outer.owner.is(Package) // if there is such an outer context, its owner should be package
+        && !location
+            .stripPrefix(outer.owner.showFullName) // p.q in p.q.r
+            .stripPrefix(".") // leading dot in .r
+            .contains(".") // remaining r so p.q.v.w leaves v.w
+      case _ =>
+        !location.contains(".") // no outer context contributes a name, effectively _root_.p.q
+    }
+
   extension (sel: ImportSelector)
     def boundTpe: Type = sel.bound match
       case untpd.TypedSplice(tree) => tree.tpe

tests/warn/i23047.scala

@@ -3,7 +3,7 @@
 package some.example:
   package demo:
 
-    import some.example
+    import some.example // no warn because enclosing package example is not available as a simple name in some
 
     object Main {
 
@@ -14,6 +14,9 @@ package some.example:
 
         import some.example.demo.Main // warn
         println(Main)
+
+        import some.example.demo // warn because enclosing package demo is available as a simple name
+        println(demo.Main)
       }
     }