From f9157802a722ac822729f27961cbe13d25565d0a Mon Sep 17 00:00:00 2001
From: Christopher Kohlhoff <chris@kohlhoff.com>
Date: Mon, 1 Dec 2025 22:49:47 +1100
Subject: [PATCH] Ensure coroutine case labels are deterministic when
 __COUNTER__ is used.

Coroutine implementations can be included from multiple translation
units in a different order. When __COUNTER__ is used (for MSVC), this
means that the values yielded by __COUNTER__ for a particular coroutine
can differ between TUs.
---
 include/boost/asio/coroutine.hpp | 38 +++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 8 deletions(-)

diff --git a/include/boost/asio/coroutine.hpp b/include/boost/asio/coroutine.hpp
index f736ea0e..6469bae5 100644
--- a/include/boost/asio/coroutine.hpp
+++ b/include/boost/asio/coroutine.hpp
@@ -276,12 +276,34 @@ private:
   bool modified_;
 };
 
+class coroutine_base_value
+{
+public:
+  constexpr coroutine_base_value(int value) : value_(value) {}
+  constexpr operator bool() const { return false; }
+  constexpr int get() const { return value_; }
+private:
+  int value_;
+};
+
 } // namespace detail
 } // namespace asio
 } // namespace boost
 
+#if !defined(BOOST_ASIO_CORO_VALUE_INIT)
+# if defined(_MSC_VER)
+#  define BOOST_ASIO_CORO_VALUE_INIT __COUNTER__
+# else // defined(_MSC_VER)
+#  define BOOST_ASIO_CORO_VALUE_INIT __LINE__
+# endif // defined(_MSC_VER)
+#endif // !defined(BOOST_ASIO_CORO_VALUE_INIT)
+
 #define BOOST_ASIO_CORO_REENTER(c) \
-  switch (::boost::asio::detail::coroutine_ref _coro_value = c) \
+  if (constexpr ::boost::asio::detail::coroutine_base_value \
+      _coro_base_value = BOOST_ASIO_CORO_VALUE_INIT) \
+  { \
+  } \
+  else switch (::boost::asio::detail::coroutine_ref _coro_value = c) \
     case -1: if (_coro_value) \
     { \
       goto terminate_coroutine; \
@@ -319,12 +341,12 @@ private:
     } \
     else
 
-#if defined(_MSC_VER)
-# define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__COUNTER__ + 1)
-# define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__COUNTER__ + 1)
-#else // defined(_MSC_VER)
-# define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__LINE__)
-# define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__LINE__)
-#endif // defined(_MSC_VER)
+# define BOOST_ASIO_CORO_YIELD \
+  BOOST_ASIO_CORO_YIELD_IMPL( \
+    BOOST_ASIO_CORO_VALUE_INIT + 1 - _coro_base_value.get())
+
+# define BOOST_ASIO_CORO_FORK \
+  BOOST_ASIO_CORO_FORK_IMPL( \
+    BOOST_ASIO_CORO_VALUE_INIT + 1 - _coro_base_value.get())
 
 #endif // BOOST_ASIO_COROUTINE_HPP