Fix stale top level synthetic package object being used in later runs (#23464)

Fixes #13821
To explain what was happening:
We previously got a completion assertion crash, because a stale symbol
(`val candidate`) was brought forward into a new run, even though it
should not have been, when searching for implicits. This was caused by
the synthetic top level package object (`file$package`, generated only
when top level definitions require it to, which is why the issue
appeared only when `type DFBits = Long` was present) passing
`stillValid` check, despite not being found in the Symbols owned by
owner (`package <empty>`). This would cause all of the stale members of
that to also be brought forward (as they would fulfill the requirements
of `stillValidInOwner`).

The stale top level package would incorrectly pass the
`(denot.is(Synthetic) && denot.is(ModuleClass) &&
stillValidInOwner(denot.companionClass))` check in stillValidInOwner,
despite companionClass of that package object being a noSymbol. Now we
also check if companionClass is a `noSymbol` explicitly to avoid this
issue.

Author: odersky

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

@@ -2720,7 +2720,7 @@ object SymDenotations {
       || owner.isRefinementClass
       || owner.is(Scala2x)
       || owner.unforcedDecls.contains(denot.name, denot.symbol)
-      || (denot.is(Synthetic) && denot.is(ModuleClass) && stillValidInOwner(denot.companionClass))
+      || (denot.is(Synthetic) && denot.is(ModuleClass) && denot.companionClass.exists && stillValidInOwner(denot.companionClass))
       || denot.isSelfSym
       || denot.isLocalDummy)
   catch case ex: StaleSymbol => false

tests/neg/i19351.check

@@ -0,0 +1,6 @@
+trait Conversions:
+  given conv(using DFBits.Candidate): Conversion[Int, DFVal] = ???
+
+import scala.quoted.*
+transparent inline def f: Short = ${ getWidthMacro }
+private def getWidthMacro(using Quotes): Expr[Short] = '{ ??? }

tests/neg/i19351/A.scala

@@ -0,0 +1,12 @@
+type DFBits = Long
+object DFBits:
+  def a: Unit = f // forces suspension of this compilation unit in typer
+  def b: DFVal = 2 // uses implicit conversion `DFVal.conv`
+
+  trait Candidate
+  object Candidate:
+    given candidate: Candidate = ??? // completed in run 3 but created in run 2
+end DFBits
+
+trait DFVal
+object DFVal extends Conversions

tests/neg/i19351a.check

@@ -0,0 +1,7 @@
+import outer._
+trait Conversions:
+  given conv(using DFBits.Candidate): Conversion[Int, DFVal] = ???
+
+import scala.quoted.*
+transparent inline def f: Short = ${ getWidthMacro }
+private def getWidthMacro(using Quotes): Expr[Short] = '{ ??? }

tests/pos-macros/i13821-a/Macro.scala

@@ -0,0 +1,14 @@
+type outer = Int
+object outer:
+  type DFBits = Long
+  object DFBits:
+    def a: Unit = f // forces suspension of this compilation unit in typer
+    def b: DFVal = 2 // uses implicit conversion `DFVal.conv`
+
+    trait Candidate
+    object Candidate:
+      given candidate: Candidate = ??? // completed in run 3 but created in run 2
+  end DFBits
+
+  trait DFVal
+  object DFVal extends Conversions

tests/pos-macros/i13821-a/Test.scala

@@ -0,0 +1,6 @@
+// object A:
+  val x: Int = 1
+  inline def myMacro(): x.type = ${myMacroExpr}
+  def test = myMacro()
+
+@main def main() = ()

tests/pos-macros/i13821-b/Macro.scala

@@ -1,3 +1,3 @@
 import scala.quoted.*
-//import A.*
+// import A.*
 def myMacroExpr(using Quotes): Expr[x.type] = '{???}