From 22f9265008b0b530e052aebf19ec324169984348 Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Tue, 21 Nov 2023 16:16:53 +0100
Subject: [PATCH] Revert part of 171773d9d8067dc54028c54c0cc8257c9c06a1a8

Fixes #19006
---
 .../dotty/tools/dotc/typer/Applications.scala |  5 +++-
 tests/pos/i19006a.scala                       | 11 ++++++++
 tests/pos/i19006b.scala                       | 27 +++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i19006a.scala
 create mode 100644 tests/pos/i19006b.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index 49c8910712c7..62cef94330c1 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/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
diff --git a/tests/pos/i19006a.scala b/tests/pos/i19006a.scala
new file mode 100644
index 000000000000..d38100997987
--- /dev/null
+++ b/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) => ???)
diff --git a/tests/pos/i19006b.scala b/tests/pos/i19006b.scala
new file mode 100644
index 000000000000..fbec70423ae9
--- /dev/null
+++ 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
+  }