Fix selecting terms using _root_

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1071,12 +1071,13 @@ object Parsers {
         nme.ERROR
       }
 
+    def checkNotRoot(name: Name): name.type =
+      if name == nme.ROOTPKG then syntaxError(em"Illegal use of root package name.")
+      name
+
     /** Accept identifier and return Ident with its name as a term name. */
     def termIdent(): Ident =
-      val t = makeIdent(in.token, in.offset, ident())
-      if t.name == nme.ROOTPKG then
-        syntaxError(em"Illegal use of root package name.")
-      t
+      makeIdent(in.token, in.offset, ident())
 
     /** Accept identifier and return Ident with its name as a type name. */
     def typeIdent(): Ident =
@@ -3601,6 +3602,13 @@ object Parsers {
         case _ =>
           first :: Nil
       }
+
+      def checkForRoot(trees: List[Tree]): Unit = for tree <- trees do tree match
+        case IdPattern(id, _) => checkNotRoot(id.name)
+        case Tuple(trees) => checkForRoot(trees)
+        case _ =>
+      checkForRoot(lhs)
+
       val tpt = typedOpt()
       val rhs =
         if tpt.isEmpty || in.token == EQUALS then
@@ -3681,7 +3689,7 @@ object Parsers {
       else {
         val mods1 = addFlag(mods, Method)
         val ident = termIdent()
-        var name = ident.name.asTermName
+        var name = checkNotRoot(ident.name).asTermName
         val paramss =
           if in.featureEnabled(Feature.clauseInterleaving) then
             // If you are making interleaving stable manually, please refer to the PR introducing it instead, section "How to make non-experimental"

tests/neg/i18020.scala

@@ -41,18 +41,18 @@ def barVal: Unit =
 
 // i18050
 package p {
-  package _root_ { // error
+  package _root_ { // not-reported
     object X // error
   }
 }
 
 // scala/bug#12508
-package _root_ { // error
+package _root_ { // ok
   class C {
     val _root_ = 42 // error
   }
 }
-package _root_.p { // error
+package _root_.p { // ok
   class C
 }
 

tests/pos/i18275.scala

@@ -0,0 +1,8 @@
+package foo
+
+enum MyEnum derives _root_.foo.Eq:
+  case One
+
+trait Eq[T]
+object Eq:
+  inline def derived[T](using m: scala.deriving.Mirror.Of[T]): Eq[T] = ???