Fix selecting terms using _root_

Author: dwijnand

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

@@ -1071,12 +1071,18 @@ object Parsers {
         nme.ERROR
       }
 
+    def checkNotRoot(tree: Tree): tree.type =
+      tree match
+        case Ident(nme.ROOTPKG) => syntaxError(em"Illegal use of root package name.")
+        case _ =>
+      tree
+
+    def rootOrTermIdent(): Ident =
+      makeIdent(in.token, in.offset, ident())
+
     /** 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
+      checkNotRoot(rootOrTermIdent())
 
     /** Accept identifier and return Ident with its name as a type name. */
     def typeIdent(): Ident =
@@ -4091,7 +4097,7 @@ object Parsers {
       val derived =
         if (isIdent(nme.derives)) {
           in.nextToken()
-          commaSeparated(() => convertToTypeId(qualId()))
+          commaSeparated(() => convertToTypeId(dotSelectors(rootOrTermIdent())))
         }
         else Nil
       possibleTemplateStart()

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] = ???