Don't constrain type variables in normalize

Fixes scala#15171

Author: odersky

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

@@ -793,7 +793,7 @@ object ProtoTypes {
               else mt.derivedLambdaType(mt.paramNames, mt.paramInfos, rt)
             case _ =>
               val ft = defn.FunctionOf(mt.paramInfos, rt)
-              if (mt.paramInfos.nonEmpty || ft <:< pt) ft else rt
+              if mt.paramInfos.nonEmpty || (ft frozen_<:< pt) then ft else rt
           }
         }
       case et: ExprType =>

tests/pos/i15171.scala

@@ -0,0 +1,13 @@
+case class Expr[+T](get: T)
+trait Ctx[F[_]]
+sealed trait Selector[F[_]]:
+  def appended(base: Expr[SelectLoop[F]]): Expr[SelectLoop[F]]
+
+ // Without Ctx[F] argument it would compile correctly
+class SelectLoop[F[_]](using Ctx[F])
+object SelectLoop:
+  def loopImpl[F[_]](ctx: Ctx[F])(caseDefs: List[Selector[F]]): Expr[Unit] =
+    // Adding explicit type :Expr[SelectLoop[F]] satifies the compiler
+    val s0 = Expr(new SelectLoop[F](using ctx))
+    val g = caseDefs.foldRight(s0)(_.appended(_))
+    Expr(())