From 0f03a7e40d681009ebbd1075306e06fed8443277 Mon Sep 17 00:00:00 2001
From: yaito3014 <ykakeyama3014@gmail.com>
Date: Wed, 4 Mar 2026 19:05:35 +0900
Subject: [PATCH] Constrain `operator<<`

---
 include/iris/alloy/io.hpp | 3 +++
 test/alloy/alloy.cpp      | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/iris/alloy/io.hpp b/include/iris/alloy/io.hpp
index 13175be17..f619bd7bb 100644
--- a/include/iris/alloy/io.hpp
+++ b/include/iris/alloy/io.hpp
@@ -9,6 +9,8 @@
     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 ==============================================================================*/
 
+#include <iris/io_fwd.hpp>
+
 #include <iris/alloy/tuple.hpp>
 
 #include <ostream>
@@ -40,6 +42,7 @@ struct tuple_ostream_impl<std::index_sequence<Is...>>
 } // detail
 
 template<class... Ts>
+    requires std::conjunction_v<req::ADL_ostreamable<Ts>...>
 std::ostream& operator<<(std::ostream& os, tuple<Ts...> const& t)
 {
     return detail::tuple_ostream_impl<std::make_index_sequence<sizeof...(Ts)>>::apply(os, t);
diff --git a/test/alloy/alloy.cpp b/test/alloy/alloy.cpp
index 80e79fa93..9062edb92 100644
--- a/test/alloy/alloy.cpp
+++ b/test/alloy/alloy.cpp
@@ -7,6 +7,8 @@
 
 #include <iris_test.hpp>
 
+#include <iris/io_fwd.hpp>
+
 #include <iris/alloy/adapted/std_tuple.hpp>
 #include <iris/alloy/adapted/std_pair.hpp>
 
@@ -499,6 +501,13 @@ TEST_CASE("utility")
 
 TEST_CASE("io")
 {
+    {
+        STATIC_CHECK(iris::req::ADL_ostreamable_v<alloy::tuple<int>>);
+
+        struct NotStreamable {};
+        STATIC_CHECK(!iris::req::ADL_ostreamable_v<NotStreamable>);
+        STATIC_CHECK(!iris::req::ADL_ostreamable_v<alloy::tuple<NotStreamable>>);
+    }
     {
         {
             std::stringstream ss;