Fix #19732: quotes.reflect.Ref incorrectly casting This to RefTree (#19930)

In the issue minimisation, `tpd.ref` returns a tpd.This(Ident(Inner$))
tree, where the inner ident is a type ident, which was unexpected.
Trying to adjust the `tpd.ref` to return a RefType for that case seems
dangerous, so I instead opted for a workaround directly inside the
QuotesImpl.

At first I wanted to reuse the `tpd.This` tree somehow (or at least to
use a correctly retyped `Ident`), but nothing I tried with it worked, so
I ended up writing the correct Ref by hand.

Fixes #19732

Author: nicolasstucki

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -468,7 +468,14 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
         withDefaultPos(tpd.ref(tp).asInstanceOf[tpd.RefTree])
       def apply(sym: Symbol): Ref =
         assert(sym.isTerm)
-        withDefaultPos(tpd.ref(sym).asInstanceOf[tpd.RefTree])
+        val refTree = tpd.ref(sym) match
+          case t @ tpd.This(ident) => // not a RefTree, so we need to work around this - issue #19732
+            // ident in `This` can be a TypeIdent of sym, so we manually prepare the ref here,
+            // knowing that the owner is actually `This`.
+            val term = Select(This(sym.owner), sym)
+            term.asInstanceOf[tpd.RefTree]
+          case other => other.asInstanceOf[tpd.RefTree]
+        withDefaultPos(refTree)
     end Ref
 
     type Ident = tpd.Ident

tests/run-macros/i19732.check

@@ -0,0 +1 @@
+Map(b -> 23)

tests/run-macros/i19732/Macro_1.scala

@@ -0,0 +1,42 @@
+// package dummy
+
+import scala.quoted.*
+
+trait Defaults[A]:
+  def defaults: Map[String, Any]
+
+object Defaults:
+  inline def derived[A <: Product]: Defaults[A] = ${ defaultsImpl[A] }
+
+  def defaultsImpl[A <: Product: Type](using Quotes): Expr[Defaults[A]] =
+    '{
+      new Defaults[A]:
+        def defaults: Map[String, Any] = ${ defaultParmasImpl[A] }
+    }
+
+def defaultParmasImpl[T](using quotes: Quotes, tpe: Type[T]): Expr[Map[String, Any]] =
+  import quotes.reflect.*
+
+  TypeRepr.of[T].classSymbol match
+    case None => '{ Map.empty[String, Any] }
+    case Some(sym) =>
+      val comp = sym.companionClass
+      val mod = Ref(sym.companionModule)
+      val names =
+        for p <- sym.caseFields if p.flags.is(Flags.HasDefault)
+        yield p.name
+      val namesExpr: Expr[List[String]] =
+        Expr.ofList(names.map(Expr(_)))
+
+      val body = comp.tree.asInstanceOf[ClassDef].body
+      val idents: List[Ref] =
+        for
+          case deff @ DefDef(name, _, _, _) <- body
+          if name.startsWith("$lessinit$greater$default")
+        yield mod.select(deff.symbol)
+      val typeArgs = TypeRepr.of[T].typeArgs
+      val identsExpr: Expr[List[Any]] =
+        if typeArgs.isEmpty then Expr.ofList(idents.map(_.asExpr))
+        else Expr.ofList(idents.map(_.appliedToTypes(typeArgs).asExpr))
+
+      '{ $namesExpr.zip($identsExpr).toMap }

tests/run-macros/i19732/Test_2.scala

@@ -0,0 +1,7 @@
+class Outer:
+  case class Inner(a: String, b: Int = 23) derives Defaults
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val outer = Outer()
+    println(summon[Defaults[outer.Inner]].defaults)