From 8c935dc996cdf2b978ce5589606f7f658ff55ed1 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 31 May 2026 14:34:01 +0300
Subject: [PATCH] Fix unwrap_and_invoke for functions returning void. Fixes
 #145.

---
 include/boost/system/unwrap_and_invoke.hpp | 25 +++++++++++++-
 test/Jamfile.v2                            |  2 ++
 test/unwrap_and_invoke3.cpp                | 40 ++++++++++++++++++++++
 3 files changed, 66 insertions(+), 1 deletion(-)
 create mode 100644 test/unwrap_and_invoke3.cpp

diff --git a/include/boost/system/unwrap_and_invoke.hpp b/include/boost/system/unwrap_and_invoke.hpp
index b18db3e..eb417b9 100644
--- a/include/boost/system/unwrap_and_invoke.hpp
+++ b/include/boost/system/unwrap_and_invoke.hpp
@@ -66,7 +66,8 @@ template<class R, class T, class E> int invoke_test( R& r, result<T, E> const& r
 
 template<class F, class... A,
     class R = decltype( compat::invoke( std::declval<F>(), detail::invoke_unwrap( std::declval<A>() )... ) ),
-    class E = detail::get_error_type<compat::remove_cvref_t<A>...>
+    class E = detail::get_error_type<compat::remove_cvref_t<A>...>,
+    class En = typename std::enable_if< !std::is_void<R>::value >::type
 >
 auto unwrap_and_invoke( F&& f, A&&... a ) -> result<R, E>
 {
@@ -82,6 +83,28 @@ auto unwrap_and_invoke( F&& f, A&&... a ) -> result<R, E>
     return compat::invoke( std::forward<F>(f), detail::invoke_unwrap( std::forward<A>(a) )... );
 }
 
+template<class F, class... A,
+    class R = decltype( compat::invoke( std::declval<F>(), detail::invoke_unwrap( std::declval<A>() )... ) ),
+    class E = detail::get_error_type<compat::remove_cvref_t<A>...>,
+    class En1 = void,
+    class En2 = typename std::enable_if< std::is_void<R>::value >::type
+>
+auto unwrap_and_invoke( F&& f, A&&... a ) -> result<R, E>
+{
+    {
+        result<void, E> r;
+
+        using Q = int[];
+        (void)Q{ detail::invoke_test( r, a )... };
+
+        if( !r ) return r.error();
+    }
+
+    compat::invoke( std::forward<F>(f), detail::invoke_unwrap( std::forward<A>(a) )... );
+
+    return {};
+}
+
 // unwrap_and_construct
 
 namespace detail
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 99619af..a7e3f65 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -228,3 +228,5 @@ run unwrap_and_construct.cpp ;
 run unwrap_and_construct2.cpp ;
 
 run detail_is_aggregate_test.cpp ;
+
+run unwrap_and_invoke3.cpp ;
diff --git a/test/unwrap_and_invoke3.cpp b/test/unwrap_and_invoke3.cpp
new file mode 100644
index 0000000..d744036
--- /dev/null
+++ b/test/unwrap_and_invoke3.cpp
@@ -0,0 +1,40 @@
+// Copyright 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/unwrap_and_invoke.hpp>
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::system;
+
+int g_x;
+
+void f( int x )
+{
+    g_x += x;
+}
+
+int main()
+{
+    {
+        g_x = 0;
+
+        result<int> a1( 1 );
+        result<void> r = unwrap_and_invoke( f, a1 );
+
+        BOOST_TEST( r );
+        BOOST_TEST_EQ( g_x, 1 );
+    }
+
+    {
+        auto ec = make_error_code( errc::invalid_argument );
+
+        result<int> a1( ec );
+        result<void> r = unwrap_and_invoke( f, a1 );
+
+        BOOST_TEST( r.has_error() ) && BOOST_TEST_EQ( r.error(), ec );
+    }
+
+    return boost::report_errors();
+}