Restore previous CheckShadowing with tweaks

Author: som-snytt

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

@@ -87,16 +87,16 @@ class CheckShadowing extends MiniPhase:
     )
 
   override def prepareForTypeDef(tree: tpd.TypeDef)(using Context): Context =
+    val sym = tree.symbol
     if tree.symbol.isAliasType then // if alias, the parent is the current symbol
       nestedTypeTraverser(tree.symbol).traverse(tree.rhs)
     if tree.symbol.is(Param) then // if param, the parent is up
-      val enclosing =
-        val owner = tree.symbol.ownersIterator.dropWhile(_.is(Param)).next()
-        if owner.isConstructor then owner.owner else owner
-      nestedTypeTraverser(enclosing).traverse(tree.rhs)(using ctx.outer)
-      shadowingDataApply(_.registerCandidate(enclosing, tree))
-    else
-      ctx
+      val owner = tree.symbol.owner
+      val parent = if (owner.isConstructor) then owner.owner else owner
+      nestedTypeTraverser(parent).traverse(tree.rhs)(using ctx.outer)
+      if isValidTypeParamOwner(tree.symbol.owner) then
+        shadowingDataApply(sd => sd.registerCandidate(parent, tree))
+    ctx
 
   override def transformPackageDef(tree: tpd.PackageDef)(using Context): tpd.Tree =
     shadowingDataApply(sd => sd.outOfScope())
@@ -112,10 +112,8 @@ class CheckShadowing extends MiniPhase:
 
   override def transformTypeDef(tree: tpd.TypeDef)(using Context): tpd.Tree =
     // Do not register for constructors the work is done for the Class owned equivalent TypeDef
-    if tree.symbol.is(Param) then
-      val owner = tree.symbol.ownersIterator.dropWhile(_.is(Param)).next()
-      if isValidTypeParamOwner(owner) then
-        shadowingDataApply(_.computeTypeParamShadowsFor(owner)(using ctx.outer))
+    if tree.symbol.is(Param) && isValidTypeParamOwner(tree.symbol.owner) then
+      shadowingDataApply(sd => sd.computeTypeParamShadowsFor(tree.symbol.owner)(using ctx.outer))
     // No need to start outer here, because the TypeDef reached here it's already the parent
     if tree.symbol.isAliasType then
       shadowingDataApply(sd => sd.computeTypeParamShadowsFor(tree.symbol)(using ctx))
@@ -140,7 +138,7 @@ class CheckShadowing extends MiniPhase:
 
     override def traverse(tree: tpd.Tree)(using Context): Unit =
       tree match
-        case t:tpd.TypeDef =>
+        case t: tpd.TypeDef =>
           val newCtx = shadowingDataApply(sd =>
             sd.registerCandidate(parent, t)
           )
@@ -156,7 +154,7 @@ class CheckShadowing extends MiniPhase:
 
     override def traverse(tree: tpd.Tree)(using Context): Unit =
       tree match
-        case t:tpd.Import =>
+        case t: tpd.Import =>
           val newCtx = shadowingDataApply(sd => sd.registerImport(t))
           traverseChildren(tree)(using newCtx)
         case _ =>
@@ -239,7 +237,7 @@ object CheckShadowing:
         val declarationScope = ctx.effectiveScope
         val res = declarationScope.lookup(symbol.name)
         res match
-          case s: Symbol if s.isType => Some(s)
+          case s: Symbol if s.isType && s != symbol => Some(s)
           case _ => lookForUnitShadowedType(symbol)(using ctx.outer)
 
     /** Register if the valDef is a private declaration that shadows an inherited field */

tests/warn/i19657-mega.scala

@@ -1,9 +1,4 @@
 //> using options -Wshadow:type-parameter-shadow -Wunused:all
 
-class F[X, M[N[X]]]:
+class F[X, M[N[X]]]: // warn
   private def x[X] = toString // warn // warn
-
-// the first is spurious
-// at 3: Type parameter X for type M shadows the type defined by type X in class F
-// at 4: Type parameter X for method x shadows the type defined by type X in class F
-// at 4: unused private member