From 75ab18c9fd9ee37a0f616dadf257b8f5dd19f30d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Mon, 8 Apr 2024 17:55:50 +0300
Subject: [PATCH] Support result<X> & fv, where fv returns void. Refs #119.

---
 include/boost/system/result.hpp | 40 ++++++++++++++++++++++++++++--
 test/result_and_fn1v.cpp        | 44 +++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+), 2 deletions(-)

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index f6a41c4..d0c8d4e 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -1124,7 +1124,8 @@ result<T, E>& operator|=( result<T, E>& r, F&& f )
 
 template<class T, class E, class F,
     class U = decltype( std::declval<F>()( std::declval<T const&>() ) ),
-    class En = typename std::enable_if<!detail::is_result<U>::value>::type
+    class En1 = typename std::enable_if<!detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<!std::is_void<U>::value>::type
 >
 result<U, E> operator&( result<T, E> const& r, F&& f )
 {
@@ -1140,7 +1141,8 @@ result<U, E> operator&( result<T, E> const& r, F&& f )
 
 template<class T, class E, class F,
     class U = decltype( std::declval<F>()( std::declval<T>() ) ),
-    class En = typename std::enable_if<!detail::is_result<U>::value>::type
+    class En1 = typename std::enable_if<!detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<!std::is_void<U>::value>::type
 >
 result<U, E> operator&( result<T, E>&& r, F&& f )
 {
@@ -1154,6 +1156,40 @@ result<U, E> operator&( result<T, E>&& r, F&& f )
     }
 }
 
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()( std::declval<T const&>() ) ),
+    class En = typename std::enable_if<std::is_void<U>::value>::type
+>
+result<U, E> operator&( result<T, E> const& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        std::forward<F>( f )( *r );
+        return {};
+    }
+}
+
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class En = typename std::enable_if<std::is_void<U>::value>::type
+>
+result<U, E> operator&( result<T, E>&& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        std::forward<F>( f )( *std::move( r ) );
+        return {};
+    }
+}
+
 template<class E, class F,
     class U = decltype( std::declval<F>()() ),
     class En = typename std::enable_if<!detail::is_result<U>::value>::type
diff --git a/test/result_and_fn1v.cpp b/test/result_and_fn1v.cpp
index 929bef5..6fb27d0 100644
--- a/test/result_and_fn1v.cpp
+++ b/test/result_and_fn1v.cpp
@@ -55,6 +55,10 @@ int k()
     return 7;
 }
 
+void fv1( int /*x*/ )
+{
+}
+
 int main()
 {
     {
@@ -199,5 +203,45 @@ int main()
         BOOST_TEST( r2.has_error() );
     }
 
+    {
+        result<int> r( 1 );
+        result<void> r2 = r & fv1;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<int> const r( 1 );
+        result<void> r2 = r & fv1;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<void> r2 = result<int>( 1 ) & fv1;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<int, E> r( in_place_error );
+        result<void, E> r2 = r & fv1;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E> const r( in_place_error );
+        result<void, E> r2 = r & fv1;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E> r2 = result<int, E>( in_place_error ) & fv1;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
     return boost::report_errors();
 }