Fix issue with static this references erroring in quoted code (#22618)

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).

Fixes #22592

Author: hamzaremmal

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

@@ -203,19 +203,27 @@ 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) }