Handle type aliases in contextFunctionResultTypeAfter

Closes #21433

Author: wjoel

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

@@ -101,8 +101,9 @@ object ContextFunctionResults:
   def contextFunctionResultTypeAfter(meth: Symbol, depth: Int)(using Context) =
     def recur(tp: Type, n: Int): Type =
       if n == 0 then tp
-      else tp match
+      else tp.dealias match
         case defn.FunctionTypeOfMethod(mt) => recur(mt.resType, n - 1)
+        case _ => tp
     recur(meth.info.finalResultType, depth)
 
   /** Should selection `tree` be eliminated since it refers to an `apply`

tests/pos/i21433.scala

@@ -0,0 +1,6 @@
+trait A[T]:
+  type R = T ?=> Unit
+  def f: R = ()
+
+class B extends A[Int]:
+  override def f: R = ()