From 5c1d0df8938661d0de87b9f8c0b0d48fa86d0b9e Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Wed, 31 Jan 2024 05:26:01 +0200
Subject: [PATCH] Enable result<void> & fv. Refs #117.

---
 include/boost/system/result.hpp | 16 ++++++++++++
 test/result_and_fn1v.cpp        | 45 +++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index 762afa3..f3d841f 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -1103,6 +1103,22 @@ result<U, E> operator&( result<T, E>&& r, F&& f )
     }
 }
 
+template<class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<!detail::is_result<U>::value>::type
+>
+result<U, E> operator&( result<void, E> const& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
 // result & unary-returning-result
 
 template<class T, class E, class F,
diff --git a/test/result_and_fn1v.cpp b/test/result_and_fn1v.cpp
index 8dac8d1..929bef5 100644
--- a/test/result_and_fn1v.cpp
+++ b/test/result_and_fn1v.cpp
@@ -50,6 +50,11 @@ int& h( int& )
     return x;
 }
 
+int k()
+{
+    return 7;
+}
+
 int main()
 {
     {
@@ -154,5 +159,45 @@ int main()
         BOOST_TEST( r2.has_error() );
     }
 
+    {
+        result<void> r;
+        result<int> r2 = r & k;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+    }
+
+    {
+        result<void> const r;
+        result<int> r2 = r & k;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+    }
+
+    {
+        result<int> r2 = result<void>() & k;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 7 );
+    }
+
+    {
+        result<void, E> r( in_place_error );
+        result<int, E> r2 = r & k;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E> const r( in_place_error );
+        result<int, E> r2 = r & k;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E> r2 = result<void, E>( in_place_error ) & k;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
     return boost::report_errors();
 }