Fix issue with static this references erroring in quoted code

Previously, inherited methods, even if accessed via static objects,
were not able to be used in quotations, unless explicitly pointed to
with a non-`this` prefix. This was due to the fact that during the
cross-stage safety check, first the method itself was checked for if it
was static (which for the inherited method, it was not), and if not,
the prefix was checked further, erroring on any `this` tree found along
the way.

This was fixed by allowing `this` trees if they point to static objects.
This way not only is the initial issue fixed, but also we are able to
freely reference static methods with `this`, like '{this.objectMethod}
(whereas previously, only '{Object.objectMethod} or '{objectMethod}
were allowed, despite them all pointing to the same static method).

[Cherry-picked 71ddfb5]

Author: jchyb

compiler/src/dotty/tools/dotc/staging/CrossStageSafety.scala

@@ -189,19 +189,27 @@ class CrossStageSafety extends TreeMapWithStages {
 
   /** Check level consistency of terms references */
   private def checkLevelConsistency(tree: Ident | This)(using Context): Unit =
+    def isStatic(pre: Type)(using Context): Boolean = pre match
+      case pre: NamedType =>
+        val sym = pre.currentSymbol
+        sym.is(Package) || sym.isStaticOwner && isStatic(pre.prefix)
+      case pre: ThisType => isStatic(pre.tref)
+      case _ => true
     new TypeTraverser {
       def traverse(tp: Type): Unit =
         tp match
           case tp @ TermRef(NoPrefix, _) if !tp.symbol.isStatic && level != levelOf(tp.symbol) =>
             levelError(tp.symbol, tp, tree.srcPos)
+          case tp: ThisType if isStatic(tp) =>
+            // static object (OK)
           case tp: ThisType if level != -1 && level != levelOf(tp.cls) =>
             levelError(tp.cls, tp, tree.srcPos)
           case tp: AnnotatedType =>
             traverse(tp.parent)
           case _ if tp.typeSymbol.is(Package) =>
             // OK
           case _ =>
-             traverseChildren(tp)
+            traverseChildren(tp)
     }.traverse(tree.tpe)
 
   private def levelError(sym: Symbol, tp: Type, pos: SrcPos)(using Context): tp.type = {

tests/neg/i22592.scala

@@ -0,0 +1,14 @@
+import scala.quoted.*
+
+trait Foo:
+  def inherited = ()
+
+class Bar extends Foo:
+  def local = ()
+  def localArg(arg: Any) = ()
+
+  def macro1(using Quotes): Expr[Unit] = '{ local } // error
+  def macro3(using Quotes): Expr[Unit] = '{ inherited } // error
+  def macro4(using Quotes): Expr[Unit] = '{ this.local } // error
+  def macro5(using Quotes): Expr[Unit] = '{ this.inherited } // error
+  def macro6(using Quotes): Expr[Unit] = '{ localArg(this) } // error // error

tests/pos/i22592.scala

@@ -0,0 +1,15 @@
+import scala.quoted.*
+
+trait Foo:
+  def inherited = ()
+
+object Bar extends Foo:
+  def local = ()
+  def localArg(arg: Any) = ()
+
+  def macro1(using Quotes): Expr[Unit] = '{ local }
+  def macro2(using Quotes): Expr[Unit] = '{ Bar.inherited }
+  def macro3(using Quotes): Expr[Unit] = '{ inherited }
+  def macro4(using Quotes): Expr[Unit] = '{ this.local }
+  def macro5(using Quotes): Expr[Unit] = '{ this.inherited }
+  def macro6(using Quotes): Expr[Unit] = '{ localArg(this) }