Use setType on shortcut for ROOTPKG

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

@@ -561,30 +561,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         return typedQuotedTypeVar(tree, pt)
     end if
 
-    // Shortcut for the root package, this is not just a performance
-    // optimization, it also avoids forcing imports thus potentially avoiding
-    // cyclic references.
-    if (name == nme.ROOTPKG)
-      return tree.withType(defn.RootPackage.termRef)
-
-    val rawType =
-      val saved1 = unimported
-      val saved2 = foundUnderScala2
-      unimported = Set.empty
-      foundUnderScala2 = NoType
-      try
-        val found = findRef(name, pt, EmptyFlags, EmptyFlags, tree.srcPos)
-        if foundUnderScala2.exists && !(foundUnderScala2 =:= found) then
-          report.migrationWarning(
-            em"""Name resolution will change.
-              | currently selected                          : $foundUnderScala2
-              | in the future, without -source 3.0-migration: $found""", tree.srcPos)
-          foundUnderScala2
-        else found
-      finally
-      	unimported = saved1
-      	foundUnderScala2 = saved2
-
     /** Normally, returns `ownType` except if `ownType` is a constructor proxy,
      *  and there is another shadowed type accessible with the same name that is not:
      *    - if the prototype is an application:
@@ -623,6 +599,31 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       checkLegalValue(tree2, pt)
       tree2
 
+    // Shortcut for the root package, this is not just a performance
+    // optimization, it also avoids forcing imports thus potentially avoiding
+    // cyclic references.
+    if (name == nme.ROOTPKG)
+      return setType(defn.RootPackage.termRef)
+
+    val rawType =
+      val saved1 = unimported
+      val saved2 = foundUnderScala2
+      unimported = Set.empty
+      foundUnderScala2 = NoType
+      try
+        val found = findRef(name, pt, EmptyFlags, EmptyFlags, tree.srcPos)
+        if foundUnderScala2.exists && !(foundUnderScala2 =:= found) then
+          report.migrationWarning(
+            em"""Name resolution will change.
+              | currently selected                          : $foundUnderScala2
+              | in the future, without -source 3.0-migration: $found""", tree.srcPos)
+          foundUnderScala2
+        else found
+      finally
+      	unimported = saved1
+      	foundUnderScala2 = saved2
+
+
     def isLocalExtensionMethodRef: Boolean = rawType match
       case rawType: TermRef =>
         rawType.denot.hasAltWith(_.symbol.is(ExtensionMethod))

tests/neg/i17757.check

@@ -0,0 +1,24 @@
+-- [E008] Not Found Error: tests/neg/i17757.scala:3:17 -----------------------------------------------------------------
+3 |  println(_root_.length)  // error
+  |          ^^^^^^^^^^^^^
+  |          value length is not a member of <root>
+-- [E119] Type Error: tests/neg/i17757.scala:4:10 ----------------------------------------------------------------------
+4 |  println(_root_)  // error
+  |          ^^^^^^
+  |          package _root_ is not a value
+-- [E119] Type Error: tests/neg/i17757.scala:7:2 -----------------------------------------------------------------------
+7 |  _root_  // error
+  |  ^^^^^^
+  |  package _root_ is not a value
+-- [E119] Type Error: tests/neg/i17757.scala:8:9 -----------------------------------------------------------------------
+8 |  _root_.scala  // error
+  |  ^^^^^^^^^^^^
+  |  package scala is not a value
+-- [E119] Type Error: tests/neg/i17757.scala:9:10 ----------------------------------------------------------------------
+9 |  println(_root_)  // error
+  |          ^^^^^^
+  |          package _root_ is not a value
+-- [E119] Type Error: tests/neg/i17757.scala:10:17 ---------------------------------------------------------------------
+10 |  println(_root_.scala)  // error
+   |          ^^^^^^^^^^^^
+   |          package scala is not a value

tests/neg/i17757.scala

@@ -0,0 +1,10 @@
+def foo: Unit =
+  val _root_  = "abc"
+  println(_root_.length)  // error
+  println(_root_)  // error
+
+def bar: Unit =
+  _root_  // error
+  _root_.scala  // error
+  println(_root_)  // error
+  println(_root_.scala)  // error