Fix references to class members defined in quotes

If an inner quote selects a symbol that is defined in an outer quote we
need to transform it or reject it. The issue is that the inner quote
cannot contain a reference to the type of the class defined in the outer
quote. Any such reference is erased the parents of that class that are
statically know outside those quotes. If the selected symbol is
overriding a symbol in one of those statically known classes, we can use
that overridden symbol instead. If not we have to reject the code.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Splicing.scala

@@ -208,6 +208,14 @@ class Splicing extends MacroTransform:
 
     override def transform(tree: tpd.Tree)(using Context): tpd.Tree =
       tree match
+        case tree: Select if tree.isTerm && isCaptured(tree.symbol) =>
+          tree.symbol.allOverriddenSymbols.find(sym => !isCaptured(sym.owner)) match
+            case Some(sym) =>
+              // virtualize call on overridden symbol that is not defined in a non static class
+              transform(tree.qualifier.select(sym))
+            case _ =>
+              report.error(em"Can not use reference to staged local ${tree.symbol} defined in an outer quote.\n\nThis can work if ${tree.symbol.owner} would extend a top level interface that defines ${tree.symbol}.", tree)
+              tree
         case tree: RefTree =>
           if tree.isTerm then
             if isCaptured(tree.symbol) then

tests/pos-macros/i17103a.scala

@@ -0,0 +1,21 @@
+import scala.quoted.*
+
+trait C0:
+  def d: Int
+
+def test(using Quotes): Expr[Unit] =
+  '{
+    trait C1 extends C0:
+      def d: Int
+    trait C extends C1:
+      def d: Int
+    val c: C = ???
+    ${
+      val expr = '{
+        val cRef: C = ???
+        cRef.d // calls C0.d
+        ()
+      }
+      expr
+    }
+  }