Enclosing package p.q not visible as q (#23069)

Fixes #23047 

Normally, an enclosing definition always has higher "name binding"
precedence than an import, but an enclosing packaging can have a dotted
name with a simple name that can't be referenced without qualification
or import.

Author: sjrd

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

@@ -370,15 +370,15 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     var cachePoint: Context = NoContext // last context with Resolved cache
     var importer: ImportSelector | Null = null // non-null for import context
     var precedence = NoPrecedence // of current resolution
+    var enclosed = false // true if sym is owner of an enclosing context
     var done = false
     var cached = false
     val ctxs = ctx.outersIterator
     while !done && ctxs.hasNext do
       val cur = ctxs.next()
-      if cur.owner eq sym then
-        addCached(cachePoint, Definition)
-        return // found enclosing definition
-      else if isLocal then
+      if cur.owner.userSymbol == sym && !sym.is(Package) then
+        enclosed = true // found enclosing definition, don't register the reference
+      if isLocal then
         if cur.owner eq sym.owner then
           done = true // for local def, just checking that it is not enclosing
       else
@@ -419,7 +419,7 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
                 candidate = cur
                 importer = sel
         else if checkMember(cur.owner) then
-          if sym.srcPos.sourcePos.source == ctx.source then
+          if sym.is(Package) || sym.srcPos.sourcePos.source == ctx.source then
             precedence = Definition
             candidate = cur
             importer = null // ignore import in same scope; we can't check nesting level
@@ -429,7 +429,8 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
             candidate = cur
     end while
     // record usage and possibly an import
-    refInfos.refs.addOne(sym)
+    if !enclosed then
+      refInfos.refs.addOne(sym)
     if candidate != NoContext && candidate.isImportContext && importer != null then
       refInfos.sels.put(importer, ())
     // possibly record that we have performed this look-up

tests/warn/i23047.scala

@@ -0,0 +1,25 @@
+//> using options -Wunused:imports
+
+package some.example:
+  package demo:
+
+    import some.example // no warn because enclosing package example is not available as a simple name in some
+
+    object Main {
+
+      def generic[T](x: Any): T = null.asInstanceOf[T]
+
+      def main(args: Array[String]): Unit = {
+        generic[example.Util](0)
+
+        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)
+      }
+    }
+
+package some.example:
+
+  class Util