Implement -Xlint:private-shadow, type-parameter-shadow (#17622)

Implemented two additional lint warnings for my bachelor project
supervised by @anatoliykmetyuk.

Most of the work has been done in a new `MiniPhase` class called
`CheckShadowing` and running in the same mega phase as the
`CheckUnused.PostTyper` mini phase.

They respectively warn about :
- A private field or a class parameter that shadows a super-class field.
- A type parameter that shadows a type already defined in the scope.

Some examples can be seen in #17612 and #17613.

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -35,7 +35,8 @@ class Compiler {
   protected def frontendPhases: List[List[Phase]] =
     List(new Parser) ::             // Compiler frontend: scanner, parser
     List(new TyperPhase) ::         // Compiler frontend: namer, typer
-    List(new CheckUnused.PostTyper) ::  // Check for unused elements
+    List(new CheckUnused.PostTyper) :: // Check for unused elements
+    List(new CheckShadowing) :: // Check shadowing elements
     List(new YCheckPositions) ::    // YCheck positions
     List(new sbt.ExtractDependencies) :: // Sends information on classes' dependencies to sbt via callbacks
     List(new semanticdb.ExtractSemanticDB) :: // Extract info into .semanticdb files

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -9,6 +9,7 @@ import dotty.tools.dotc.config.SourceVersion
 import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.rewrites.Rewrites
 import dotty.tools.io.{AbstractFile, Directory, JDK9Reflectors, PlainDirectory}
+import Setting.ChoiceWithHelp
 
 import scala.util.chaining._
 
@@ -156,7 +157,6 @@ private sealed trait VerboseSettings:
  */
 private sealed trait WarningSettings:
   self: SettingGroup =>
-  import Setting.ChoiceWithHelp
 
   val Whelp: Setting[Boolean] = BooleanSetting("-W", "Print a synopsis of warning options.")
   val XfatalWarnings: Setting[Boolean] = BooleanSetting("-Werror", "Fail the compilation if there are any warnings.", aliases = List("-Xfatal-warnings"))
@@ -307,6 +307,27 @@ private sealed trait XSettings:
   }
 
   val XmacroSettings: Setting[List[String]] = MultiStringSetting("-Xmacro-settings", "setting1,setting2,..settingN", "List of settings which exposed to the macros")
+
+  val Xlint: Setting[List[ChoiceWithHelp[String]]] = UncompleteMultiChoiceHelpSetting(
+    name = "-Xlint",
+    helpArg = "advanced warning",
+    descr = "Enable or disable specific `lint` warnings",
+    choices = List(
+      ChoiceWithHelp("all", ""),
+      ChoiceWithHelp("private-shadow", "Warn if a private field or class parameter shadows a superclass field"),
+      ChoiceWithHelp("type-parameter-shadow", "Warn when a type parameter shadows a type already in the scope"),
+    ),
+    default = Nil
+  )
+
+  object XlintHas:
+    def allOr(s: String)(using Context) =
+      Xlint.value.pipe(us => us.contains("all") || us.contains(s))
+    def privateShadow(using Context) =
+      allOr("private-shadow")
+    def typeParameterShadow(using Context) =
+      allOr("type-parameter-shadow")
+
 end XSettings
 
 /** -Y "Forking" as in forked tongue or "Private" settings */

compiler/src/dotty/tools/dotc/config/Settings.scala

@@ -62,6 +62,7 @@ object Settings:
     prefix: String = "",
     aliases: List[String] = Nil,
     depends: List[(Setting[?], Any)] = Nil,
+    ignoreInvalidArgs: Boolean = false,
     propertyClass: Option[Class[?]] = None)(private[Settings] val idx: Int) {
 
     private var changed: Boolean = false
@@ -104,8 +105,16 @@ object Settings:
       def fail(msg: String, args: List[String]) =
         ArgsSummary(sstate, args, errors :+ msg, warnings)
 
+      def warn(msg: String, args: List[String]) =
+        ArgsSummary(sstate, args, errors, warnings :+ msg)
+
       def missingArg =
-        fail(s"missing argument for option $name", args)
+        val msg = s"missing argument for option $name"
+        if ignoreInvalidArgs then warn(msg + ", the tag was ignored", args)  else fail(msg, args)
+
+      def invalidChoices(invalid: List[String]) =
+        val msg = s"invalid choice(s) for $name: ${invalid.mkString(",")}"
+        if ignoreInvalidArgs then warn(msg + ", the tag was ignored", args) else fail(msg, args)
 
       def setBoolean(argValue: String, args: List[String]) =
         if argValue.equalsIgnoreCase("true") || argValue.isEmpty then update(true, args)
@@ -144,7 +153,7 @@ object Settings:
             choices match
               case Some(valid) => strings.filterNot(valid.contains) match
                 case Nil => update(strings, args)
-                case invalid => fail(s"invalid choice(s) for $name: ${invalid.mkString(",")}", args)
+                case invalid => invalidChoices(invalid)
               case _ => update(strings, args)
         case (StringTag, _) if argRest.nonEmpty || choices.exists(_.contains("")) =>
           setString(argRest, args)
@@ -287,6 +296,9 @@ object Settings:
     def MultiChoiceHelpSetting(name: String, helpArg: String, descr: String, choices: List[ChoiceWithHelp[String]], default: List[ChoiceWithHelp[String]], aliases: List[String] = Nil): Setting[List[ChoiceWithHelp[String]]] =
       publish(Setting(name, descr, default, helpArg, Some(choices), aliases = aliases))
 
+    def UncompleteMultiChoiceHelpSetting(name: String, helpArg: String, descr: String, choices: List[ChoiceWithHelp[String]], default: List[ChoiceWithHelp[String]], aliases: List[String] = Nil): Setting[List[ChoiceWithHelp[String]]] =
+      publish(Setting(name, descr, default, helpArg, Some(choices), aliases = aliases, ignoreInvalidArgs = true))
+
     def IntSetting(name: String, descr: String, default: Int, aliases: List[String] = Nil): Setting[Int] =
       publish(Setting(name, descr, default, aliases = aliases))