Fix selecting terms using _root_ (#18335)

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

@@ -1073,10 +1073,7 @@ object Parsers {
 
     /** 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 =

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -968,6 +968,11 @@ trait Checking {
         em"Implementation restriction: ${path.tpe.classSymbol} is not a valid prefix for a wildcard export, as it is a package",
         path.srcPos)
 
+  /** Check that the definition name isn't root. */
+  def checkNonRootName(name: Name, nameSpan: Span)(using Context): Unit =
+    if name == nme.ROOTPKG then
+      report.error(em"Illegal use of root package name.", ctx.source.atSpan(nameSpan))
+
   /** Check that module `sym` does not clash with a class of the same name
    *  that is concurrently compiled in another source file.
    */

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2472,6 +2472,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
   def typedValDef(vdef: untpd.ValDef, sym: Symbol)(using Context): Tree = {
     val ValDef(name, tpt, _) = vdef
+    checkNonRootName(vdef.name, vdef.nameSpan)
     completeAnnotations(vdef, sym)
     if (sym.isOneOf(GivenOrImplicit)) checkImplicitConversionDefOK(sym)
     if sym.is(Module) then checkNoModuleClash(sym)
@@ -2505,6 +2506,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       // hence we special case it until `erased` is no longer experimental.
       sym.setFlag(Erased)
     val DefDef(name, paramss, tpt, _) = ddef
+    checkNonRootName(ddef.name, ddef.nameSpan)
     completeAnnotations(ddef, sym)
     val paramss1 = paramss.nestedMapConserve(typed(_)).asInstanceOf[List[ParamClause]]
     for case ValDefs(vparams) <- paramss1 do
@@ -2853,6 +2855,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val pkg = pid1.symbol
     pid1 match
       case pid1: RefTree if pkg.is(Package) =>
+        if ctx.owner != defn.RootClass         // valid top-level "package _root_"
+        && ctx.owner != defn.EmptyPackageClass // valid "package _root_" after parser's "package <empty>" wrapper
+        then
+          checkNonRootName(pid1.name, pid1.span)
         inContext(ctx.packageContext(tree, pkg)) {
           // If it exists, complete the class containing the top-level definitions
           // before typing any statement in the package to avoid cycles as in i13669.scala

tests/neg/i18020.scala

@@ -47,12 +47,12 @@ package p {
 }
 
 // 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] = ???