Deprecate nested class shadowing

Fixes scala/bug#8353
Fixes scala/bug#11360

This deprecates nested class shadowing for Dotty compatibilty.

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -769,6 +769,23 @@ trait TypeDiagnostics {
           |If applicable, you may wish to try moving some members into another object.""".stripMargin
     }
 
+    def warnNestedClassShadow(sym: Symbol): Unit =
+      if (!isPastTyper && !sym.isSynthetic && sym.isNestedClass) {
+        def enclClass(c: Context): Context =
+          if (!c.owner.exists || c.owner.isClass) c
+          else enclClass(c.outer)
+        val outerOwner = enclClass(context).outer.owner
+        outerOwner.info.baseClasses
+          .filter(x => x != outerOwner && x != AnyRefClass && x != AnyClass && x != ObjectClass)
+          .foreach(base => {
+            val sym2 = base.info.decl(sym.name)
+            if (sym2 != NoSymbol && sym2 != sym) {
+              val msg = s"class shadowing is deprecated but class ${sym.name} shadows $sym2 defined in ${sym2.owner}; rename the class to something else"
+              context.deprecationWarning(sym.pos, sym, msg, "2.13.2")
+            }
+          })
+      }
+
     // warn about class/method/type-members' type parameters that shadow types already in scope
     def warnTypeParameterShadow(tparams: List[TypeDef], sym: Symbol): Unit =
       if (settings.warnTypeParameterShadow && !isPastTyper && !sym.isSynthetic) {

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1900,6 +1900,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           checkEphemeral(clazz, impl2.body)
 
         warnTypeParameterShadow(tparams1, clazz)
+        warnNestedClassShadow(clazz)
 
         if (!isPastTyper) {
           for (ann <- clazz.getAnnotation(DeprecatedAttr)) {

test/files/neg/nested-class-shadowing.check

@@ -0,0 +1,6 @@
+nested-class-shadowing.scala:9: warning: class shadowing is deprecated but class Status shadows class Status defined in trait Core; rename the class to something else
+  class Status extends super.Status
+        ^
+error: No warnings can be incurred under -Werror.
+1 warning
+1 error

test/files/neg/nested-class-shadowing.scala

@@ -0,0 +1,10 @@
+// scalac: -Werror -Xlint:deprecation
+//
+
+trait Core {
+  class Status
+}
+
+trait Ext extends Core {
+  class Status extends super.Status
+}