Guard against invalid prefixes in argForParam

Fixes scala#23504

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2682,8 +2682,11 @@ object Types extends TypeUtils {
           while (tparams.nonEmpty && args.nonEmpty) {
             if (tparams.head.eq(tparam))
               return args.head match {
-                case _: TypeBounds if !widenAbstract => TypeRef(pre, tparam)
-                case arg => arg.boxedUnlessFun(tycon)
+                case _: TypeBounds if !widenAbstract =>
+                  if !NamedType.validPrefix(pre) then
+                    throw TypeError(em"invalid prefix $pre cannot replace parameter $tparam in result of selection")
+                  TypeRef(pre, tparam)
+                case arg => arg
               }
             tparams = tparams.tail
             args = args.tail

tests/neg/i23504.scala

@@ -0,0 +1,2 @@
+def test =
+  Seq.empty[[T] =>> () => ?].head() // error