From 7f1cd609999e63b733c36f25d8d4a68b817ec9f3 Mon Sep 17 00:00:00 2001
From: Lixin Wei <lixin.wei@metabit-trading.com>
Date: Fri, 16 Jan 2026 15:09:04 +0800
Subject: [PATCH] fix 8-element tuple compile

---
 include/stdexec/__detail/__tuple.hpp          | 10 +++----
 test/stdexec/algos/adaptors/test_when_all.cpp | 30 +++++++++++++++++++
 2 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/include/stdexec/__detail/__tuple.hpp b/include/stdexec/__detail/__tuple.hpp
index 30ad891fa..41fd937b2 100644
--- a/include/stdexec/__detail/__tuple.hpp
+++ b/include/stdexec/__detail/__tuple.hpp
@@ -345,7 +345,11 @@ namespace STDEXEC {
     constexpr auto __size = STDEXEC_REMOVE_REFERENCE(_Tuple)::__size;
     static_assert(_Index < __size, "Index out of bounds in __get");
 
-    if constexpr (_Index == 0) {
+    // Only use __valN accessors for tuples with <= 8 elements (which have specialized storage)
+    // Larger tuples use __box base class and need __tup::__get
+    if constexpr (__size > 8) {
+      return __tup::__get<_Index>(static_cast<_Tuple&&>(__tupl));
+    } else if constexpr (_Index == 0) {
       return static_cast<_Tuple&&>(__tupl).__val0;
     } else if constexpr (_Index == 1) {
       return static_cast<_Tuple&&>(__tupl).__val1;
@@ -361,10 +365,6 @@ namespace STDEXEC {
       return static_cast<_Tuple&&>(__tupl).__val6;
     } else if constexpr (_Index == 7) {
       return static_cast<_Tuple&&>(__tupl).__val7;
-    } else if constexpr (_Index == 8) {
-      return static_cast<_Tuple&&>(__tupl).__val8;
-    } else {
-      return __tup::__get<_Index>(static_cast<_Tuple&&>(__tupl));
     }
   }
 
diff --git a/test/stdexec/algos/adaptors/test_when_all.cpp b/test/stdexec/algos/adaptors/test_when_all.cpp
index d71f6c888..acc9994fa 100644
--- a/test/stdexec/algos/adaptors/test_when_all.cpp
+++ b/test/stdexec/algos/adaptors/test_when_all.cpp
@@ -62,6 +62,36 @@ namespace {
     wait_for_value(std::move(snd), 2, 3, 5, 7, 11);
   }
 
+  TEST_CASE("when_all with 8 senders", "[adaptors][when_all]") {
+    // 8 senders is the boundary case for the optimized tuple specializations
+    ex::sender auto snd = ex::when_all(
+      ex::just(2),
+      ex::just(3),
+      ex::just(5),
+      ex::just(7),
+      ex::just(11),
+      ex::just(13),
+      ex::just(17),
+      ex::just(19));
+    wait_for_value(std::move(snd), 2, 3, 5, 7, 11, 13, 17, 19);
+  }
+
+  TEST_CASE("when_all with 10 senders", "[adaptors][when_all]") {
+    // 10 senders uses the generic tuple with __box storage
+    ex::sender auto snd = ex::when_all(
+      ex::just(2),
+      ex::just(3),
+      ex::just(5),
+      ex::just(7),
+      ex::just(11),
+      ex::just(13),
+      ex::just(17),
+      ex::just(19),
+      ex::just(23),
+      ex::just(29));
+    wait_for_value(std::move(snd), 2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
+  }
+
   TEST_CASE("when_all with just one sender", "[adaptors][when_all]") {
     ex::sender auto snd = ex::when_all(ex::just(2));
     wait_for_value(std::move(snd), 2);