From 62c7d224073511050046ff1d312424203f997909 Mon Sep 17 00:00:00 2001
From: Robert Leahy <rleahy@rleahy.ca>
Date: Sat, 21 Feb 2026 13:28:31 -0500
Subject: [PATCH] exec::repeat_until et al.: Dependent Child Sender Support

The TODO eliminated by this commit not only meant that exec::
repeat_until et al. previously didn't the usual constant evaluation
techniques to report errors, it also meant that a hard compile error
(rather than a return type indicating that the sender is dependent)
resulted if:

- The child sender was dependent, and
- An attempt was made to determine the completion signatures of the
  parent without an environment

Updated so exec::repeat_until et al. now properly report that they are
dependent if their child is dependent.
---
 include/exec/repeat_until.hpp   | 19 +++++++++++++++++--
 test/exec/test_repeat_until.cpp | 19 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/include/exec/repeat_until.hpp b/include/exec/repeat_until.hpp
index 189f76108..e5f6d5d8d 100644
--- a/include/exec/repeat_until.hpp
+++ b/include/exec/repeat_until.hpp
@@ -255,13 +255,28 @@ namespace experimental::execution
         __delete_set_value_t>,
       __mbind_front_q<__values_t, _Child>::template __f>;
 
+    struct _FAILED_TO_FORM_COMPLETION_SIGNATURES
+    {};
+
     struct __repeat_until_impl : __sexpr_defaults
     {
       template <class _Sender, class... _Env>
       static consteval auto __get_completion_signatures()
       {
-        // TODO: port this to use constant evaluation
-        return __completions_t<__child_of<_Sender>, _Env...>{};
+        using __child_t = __child_of<_Sender>;
+        if constexpr (::STDEXEC::__minvocable_q<__completions_t, __child_t, _Env...>)
+        {
+          return __completions_t<__child_t, _Env...>{};
+        }
+        else if constexpr (sizeof...(_Env) == 0)
+        {
+          return STDEXEC::__dependent_sender<_Sender>();
+        }
+        else
+        {
+          return ::STDEXEC::__throw_compile_time_error<_FAILED_TO_FORM_COMPLETION_SIGNATURES,
+                                                       ::STDEXEC::_WITH_PRETTY_SENDER_<_Sender>>();
+        }
       };
 
       static constexpr auto __connect =
diff --git a/test/exec/test_repeat_until.cpp b/test/exec/test_repeat_until.cpp
index 9401f6b94..95bfd52d0 100644
--- a/test/exec/test_repeat_until.cpp
+++ b/test/exec/test_repeat_until.cpp
@@ -28,9 +28,11 @@
 #include <catch2/catch.hpp>
 
 #include <concepts>
+#include <cstddef>
 #include <limits>
 #include <memory>
 #include <stdexcept>
+#include <type_traits>
 #include <utility>
 
 namespace ex = STDEXEC;
@@ -407,4 +409,21 @@ namespace {
         "Missing added set_error_t(std::exception_ptr)");
     }
   }
+
+  TEST_CASE("repeat_until works with a dependent sender", "[adaptors][repeat_until]")
+  {
+    std::size_t invoked = 0;
+    auto        snd     = exec::repeat_until(ex::read_env(ex::get_stop_token)
+                                  | ex::then(
+                                    [&](auto token) noexcept
+                                    {
+                                      ++invoked;
+                                      return std::is_same_v<ex::never_stop_token, decltype(token)>;
+                                    }));
+    static_assert(ex::dependent_sender<decltype(snd)>);
+    auto op = ex::connect(std::move(snd), expect_void_receiver{});
+    CHECK(!invoked);
+    ex::start(op);
+    CHECK(invoked == 1);
+  }
 } // namespace