From c0988e263b4f8602c4b7bae8987ea9bda941dcce Mon Sep 17 00:00:00 2001
From: Michael Vandeberg <mvandeberg@users.noreply.github.com>
Date: Wed, 1 Apr 2026 15:39:32 -0600
Subject: [PATCH] Align Executor concept and executor_ref with P4003

- Executor concept: pass continuation by reference in requires-expression
  to match stable-address semantics of all implementations
- executor_ref constructor: constrain to Executor to reject non-conforming
  types at the call site instead of deferring to vtable instantiation
- executor_ref::target(): add noexcept to both overloads (trivially
  non-throwing type-id comparison and static_cast)
---
 include/boost/capy/concept/executor.hpp | 2 +-
 include/boost/capy/ex/executor_ref.hpp  | 8 +++++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/boost/capy/concept/executor.hpp b/include/boost/capy/concept/executor.hpp
index d7582d9ff..cf2b1ac95 100644
--- a/include/boost/capy/concept/executor.hpp
+++ b/include/boost/capy/concept/executor.hpp
@@ -166,7 +166,7 @@ template<class E>
 concept Executor =
     std::is_nothrow_copy_constructible_v<E> &&
     std::is_nothrow_move_constructible_v<E> &&
-    requires(E& e, E const& ce, E const& ce2, continuation c) {
+    requires(E& e, E const& ce, E const& ce2, continuation& c) {
         { ce == ce2 } noexcept -> std::convertible_to<bool>;
         { ce.context() } noexcept;
         requires std::is_lvalue_reference_v<decltype(ce.context())> &&
diff --git a/include/boost/capy/ex/executor_ref.hpp b/include/boost/capy/ex/executor_ref.hpp
index f6010b7bd..245f16871 100644
--- a/include/boost/capy/ex/executor_ref.hpp
+++ b/include/boost/capy/ex/executor_ref.hpp
@@ -12,6 +12,7 @@
 
 #include <boost/capy/detail/config.hpp>
 #include <boost/capy/detail/type_id.hpp>
+#include <boost/capy/concept/executor.hpp>
 #include <boost/capy/continuation.hpp>
 #include <concepts>
 #include <coroutine>
@@ -149,7 +150,8 @@ class executor_ref
             std::decay_t<Ex>, executor_ref>, int> = 0>
 #else
     template<class Ex>
-        requires (!std::same_as<std::decay_t<Ex>, executor_ref>)
+        requires (!std::same_as<std::decay_t<Ex>, executor_ref>
+            && Executor<Ex>)
 #endif
     executor_ref(Ex const& ex) noexcept
         : ex_(&ex)
@@ -266,7 +268,7 @@ class executor_ref
             `nullptr` if the type does not match.
     */
     template< typename Executor >
-    const Executor* target() const
+    const Executor* target() const noexcept
     {
         if ( *vt_->type_id == detail::type_id< Executor >() )
            return static_cast< Executor const* >( ex_ );
@@ -275,7 +277,7 @@ class executor_ref
 
     /// @copydoc target() const
     template< typename Executor>
-    Executor* target()
+    Executor* target() noexcept
     {
         if ( *vt_->type_id == detail::type_id< Executor >() )
            return const_cast< Executor* >(