Separate CheckUnused and CheckShadowing into two MegaPhases

[Cherry-picked 0f4f7bf]

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

@@ -35,10 +35,11 @@ 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 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
-    List(new CheckUnused.PostTyper, new CheckShadowing) :: // Check for unused elements and shadowing elements
     List(new PostTyper) ::          // Additional checks and cleanups after type checking
     List(new sjs.PrepJSInterop) ::  // Additional checks and transformations for Scala.js (Scala.js only)
     List(new sbt.ExtractAPI) ::     // Sends a representation of the API of classes to sbt via callbacks

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

@@ -645,17 +645,6 @@ object CheckUnused:
         imp.expr.tpe.member(sel.name.toTypeName).alternatives.exists(_.symbol.isOneOf(GivenOrImplicit))
       )
 
-    /** Returns some inherited symbol with the same type and name as the given "symDecl" */
-    private def lookForInheritedDecl(symDecl: Symbol)(using Context): Option[Symbol] =
-      val symDeclType = symDecl.info
-      val bClasses = symDecl.owner.info.baseClasses
-      bClasses match
-        case _ :: inherited =>
-          inherited
-            .map(classSymbol => symDecl.denot.matchingDecl(classSymbol, symDeclType))
-            .find(sym => sym.name == symDecl.name)
-        case Nil =>
-          None
 
 
     extension (tree: ImportSelector)
@@ -732,7 +721,7 @@ object CheckUnused:
 
       /** A function is overriden. Either has `override flags` or parent has a matching member (type and name) */
       private def isOverriden(using Context): Boolean =
-        sym.is(Flags.Override) || lookForInheritedDecl(sym).isDefined
+        sym.is(Flags.Override) || (sym.exists && sym.owner.thisType.parents.exists(p => sym.matchingMember(p).exists))
 
     end extension