From f28da1c40c9a4653f6dec62510487c1c139ac909 Mon Sep 17 00:00:00 2001
From: Eric Niebler <eniebler@nvidia.com>
Date: Mon, 6 Apr 2026 17:29:34 -0700
Subject: [PATCH] be explicit in `into_variant` about which variant alternative
 is being initialized

fixes #1988
---
 include/stdexec/__detail/__into_variant.hpp   |  5 +++--
 include/stdexec/__detail/__when_all.hpp       | 14 +++++++++++---
 test/stdexec/algos/adaptors/test_when_all.cpp | 10 ++++++++++
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/include/stdexec/__detail/__into_variant.hpp b/include/stdexec/__detail/__into_variant.hpp
index 0f011bf8b..adc52c703 100644
--- a/include/stdexec/__detail/__into_variant.hpp
+++ b/include/stdexec/__detail/__into_variant.hpp
@@ -83,8 +83,9 @@ namespace STDEXEC
           STDEXEC_TRY
           {
             STDEXEC::set_value(static_cast<_State&&>(__state).__rcvr_,
-                               __variant_t{
-                                 std::tuple<_Args&&...>{static_cast<_Args&&>(__args)...}});
+                               __variant_t{std::in_place_type<__decayed_std_tuple<_Args...>>,
+                                           std::tuple<_Args&&...>{
+                                             static_cast<_Args&&>(__args)...}});
           }
           STDEXEC_CATCH_ALL
           {
diff --git a/include/stdexec/__detail/__when_all.hpp b/include/stdexec/__detail/__when_all.hpp
index 97a8b13c6..a11e82eaf 100644
--- a/include/stdexec/__detail/__when_all.hpp
+++ b/include/stdexec/__detail/__when_all.hpp
@@ -56,6 +56,16 @@ namespace STDEXEC
 
   struct when_all_with_variant_t
   {
+   private:
+    static constexpr auto __mk_transform_fn() noexcept
+    {
+      return []<class... _Child>(__ignore, __ignore, _Child&&... __child)
+      {
+        return when_all(into_variant(static_cast<_Child&&>(__child))...);
+      };
+    }
+
+   public:
     template <sender... _Senders>
     constexpr auto operator()(_Senders&&... __sndrs) const -> __well_formed_sender auto
     {
@@ -66,9 +76,7 @@ namespace STDEXEC
     static constexpr auto transform_sender(set_value_t, _Sender&& __sndr, __ignore)
     {
       // transform when_all_with_variant(sndrs...) into when_all(into_variant(sndrs)...).
-      return __apply([&]<class... _Child>(__ignore, __ignore, _Child&&... __child)
-                     { return when_all(into_variant(static_cast<_Child&&>(__child))...); },
-                     static_cast<_Sender&&>(__sndr));
+      return __apply(__mk_transform_fn(), static_cast<_Sender&&>(__sndr));
     }
   };
 
diff --git a/test/stdexec/algos/adaptors/test_when_all.cpp b/test/stdexec/algos/adaptors/test_when_all.cpp
index 0316806e5..631eb304e 100644
--- a/test/stdexec/algos/adaptors/test_when_all.cpp
+++ b/test/stdexec/algos/adaptors/test_when_all.cpp
@@ -15,6 +15,7 @@
  */
 
 #include <catch2/catch.hpp>
+#include <exec/any_sender_of.hpp>
 #include <exec/async_scope.hpp>
 #include <exec/env.hpp>
 #include <exec/sender_for.hpp>
@@ -461,4 +462,13 @@ namespace
     scope.request_stop();
     ex::sync_wait(scope.on_empty());
   }
+
+  TEST_CASE("regression test for #1988", "[adaptors][when_all")
+  {
+    // just test that this compiles:
+    using any_sender = exec::any_sender<
+      exec::any_receiver<ex::completion_signatures<ex::set_value_t(int), ex::set_value_t(double)>>>;
+    ex::sync_wait(ex::when_all_with_variant(any_sender{ex::just(1)}));
+    SUCCEED();
+  }
 }  // namespace