Reject recursive dynamicSelect corner case

Author: odersky

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

@@ -818,14 +818,20 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     // and the selector is neither applied nor assigned to,
     // expand to a typed dynamic dispatch using selectDynamic wrapped in a cast
     if qual.tpe.derivesFrom(defn.SelectableClass) && !isDynamicExpansion(tree)
-      && !pt.isInstanceOf[FunOrPolyProto] && pt != LhsProto
+        && !pt.isInstanceOf[FunOrPolyProto] && pt != LhsProto
     then
       val fieldsType = qual.tpe.select(tpnme.Fields).dealias.simplified
       val fields = fieldsType.namedTupleElementTypes
       typr.println(i"try dyn select $qual, $selName, $fields")
       fields.find(_._1 == selName) match
         case Some((_, fieldType)) =>
-          return dynamicSelect(fieldType).ensureConforms(fieldType)
+          val dynSelected = dynamicSelect(fieldType)
+          dynSelected match
+            case Apply(sel: Select, _) if !sel.denot.symbol.exists =>
+              // Reject corner case where selectDynamic needs annother selectDynamic to be called. E.g. as in neg/unselectable-fields.scala.
+              report.error(i"Cannot use selectDynamic here since it it needs another selectDynamic to be invoked", tree.srcPos)
+            case _ =>
+          return dynSelected.ensureConforms(fieldType)
         case _ =>
 
     // Otherwise, report an error

tests/neg/unselectable-fields.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/unselectable-fields.scala:4:13 ---------------------------------------------------------------------
+4 |val _ = foo1.xyz // error
+  |        ^^^^^^^^
+  |        Cannot use selectDynamic here since it it needs another selectDynamic to be invoked

tests/pos/selectable-fields.scala renamed to tests/neg/unselectable-fields.scala

@@ -1,3 +1,6 @@
 val foo1 = new Selectable:
   type Fields = (xyz: Int)
   def selectDynamic(name: String): Any = 23
+val _ = foo1.xyz // error
+
+