Revert part of Simplify defn.FunctionOf.unapply (#19012)

See
[171773d](171773d#diff-8893c00063ce469a750bdbdbca40265550e0901a39a8a45e2f83aa6d51230aa0L2006-R2006)

Fixes #19006

Author: nicolasstucki

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

@@ -2003,7 +2003,10 @@ trait Applications extends Compatibility {
         // the arity of that function, otherise -1.
         def paramCount(ref: TermRef) =
           val formals = ref.widen.firstParamTypes
-          if formals.length > idx then defn.functionArity(formals(idx))
+          if formals.length > idx then
+            formals(idx).dealias match
+              case defn.FunctionNOf(args, _, _) => args.length
+              case _ => -1
           else -1
 
         val numArgs = args.length

tests/pos/i19006a.scala

@@ -0,0 +1,11 @@
+import java.util.Map.Entry;
+import java.util.function.BiConsumer;
+import java.lang.Iterable
+
+trait HttpHeaders extends Iterable[Entry[String, String]] {
+  def forEach(action: BiConsumer[String, String]): Unit = ???
+}
+
+@main def Test =
+  val headers: HttpHeaders = ???
+  headers.forEach((a, b) => ???)

tests/pos/i19006b.scala

@@ -0,0 +1,27 @@
+import java.util.function.Function
+
+trait HttpClient
+trait HttpRequest
+trait HttpResponse
+trait ClientRequestContext
+
+trait DecoratingHttpClientFunction {
+  def execute(delegate: HttpClient, ctx: ClientRequestContext, req: HttpRequest): HttpResponse
+}
+
+class AbstractClientOptionsBuilder:
+  def decorator(fn: Function[? <: HttpClient, ? <: HttpClient]): AbstractClientOptionsBuilder = ???
+  def decorator(fn: DecoratingHttpClientFunction): AbstractClientOptionsBuilder = ???
+
+class WebClientBuilder extends AbstractClientOptionsBuilder:
+  override def decorator(fn: Function[? <: HttpClient, ? <: HttpClient]): WebClientBuilder = ???
+  override def decorator(fn: DecoratingHttpClientFunction): WebClientBuilder = ???
+
+class ArmeriaClientBuilder[F[_]]:
+  type DecoratingFunction = (HttpClient, ClientRequestContext, HttpRequest) => HttpResponse
+  def clientBuilder: WebClientBuilder = ???
+
+  def withDecorator(decorator: DecoratingFunction): ArmeriaClientBuilder[F] = {
+    clientBuilder.decorator(decorator(_, _, _))
+    this
+  }