From d11dd4b3962349b46401479e456c43192b704c9d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 31 Oct 2023 03:08:02 +0200
Subject: [PATCH] Add operator&=( result&, unary-returning-result )

---
 include/boost/system/result.hpp |  18 +++++
 test/CMakeLists.txt             |   1 +
 test/Jamfile.v2                 |   1 +
 test/result_and_eq_fn1r.cpp     | 126 ++++++++++++++++++++++++++++++++
 4 files changed, 146 insertions(+)
 create mode 100644 test/result_and_eq_fn1r.cpp

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index bcc68581d..833254cb1 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -1137,6 +1137,24 @@ result<T, E>& operator&=( result<T, E>& r, F&& f )
     return r;
 }
 
+// result &= unary-returning-result
+
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<detail::is_value_convertible_to<typename U::value_type, T>::value>::type,
+    class En3 = typename std::enable_if<std::is_convertible<typename U::error_type, E>::value>::type
+>
+result<T, E>& operator&=( result<T, E>& r, F&& f )
+{
+    if( r )
+    {
+        r = std::forward<F>( f )( *std::move( r ) );
+    }
+
+    return r;
+}
+
 } // namespace system
 } // namespace boost
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4d75ca52c..ada24895b 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -175,3 +175,4 @@ boost_test(TYPE run SOURCES result_or_fn0r.cpp)
 boost_test(TYPE run SOURCES result_and_fn1v.cpp)
 boost_test(TYPE run SOURCES result_and_fn1r.cpp)
 boost_test(TYPE run SOURCES result_and_eq_fn1v.cpp)
+boost_test(TYPE run SOURCES result_and_eq_fn1r.cpp)
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index bac182966..4c3dcb845 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -205,3 +205,4 @@ run result_or_fn0r.cpp : : : $(CPP11) ;
 run result_and_fn1v.cpp : : : $(CPP11) ;
 run result_and_fn1r.cpp : : : $(CPP11) ;
 run result_and_eq_fn1v.cpp : : : $(CPP11) ;
+run result_and_eq_fn1r.cpp : : : $(CPP11) ;
diff --git a/test/result_and_eq_fn1r.cpp b/test/result_and_eq_fn1r.cpp
new file mode 100644
index 000000000..c9519d7d6
--- /dev/null
+++ b/test/result_and_eq_fn1r.cpp
@@ -0,0 +1,126 @@
+// Copyright 2017, 2021, 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::system;
+
+struct X
+{
+    int v_;
+};
+
+struct Y
+{
+    int v_;
+
+    explicit Y( int v ): v_( v ) {}
+    Y( X x ): v_( x.v_) {}
+
+    Y( Y const& ) = delete;
+    Y& operator=( Y const& ) = delete;
+
+    Y( Y&& r ): v_( r.v_ )
+    {
+        r.v_ = 0;
+    }
+
+    Y& operator=( Y&& r )
+    {
+        if( &r != this )
+        {
+            v_ = r.v_;
+            r.v_ = 0;
+        }
+
+        return *this;
+    }
+};
+
+struct E
+{
+};
+
+struct E2
+{
+    E2() {}
+    E2( E ) {}
+};
+
+result<int, E> fi( int x )
+{
+    return 2 * x + 1;
+}
+
+result<int, E2> fi2( int )
+{
+    return E2();
+}
+
+result<X, E> fy( Y y )
+{
+    return X{ 2 * y.v_ + 1 };
+}
+
+result<Y, E2> fy2( Y )
+{
+    return E2();
+}
+
+result<int&, E> fri( int& )
+{
+    static int x = 2;
+    return x;
+}
+
+result<int&, E2> fri2( int& )
+{
+    return E2();
+}
+
+int main()
+{
+    {
+        result<int, E2> r( 1 );
+
+        r &= fi;
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, 3 );
+
+        r &= fi2;
+        BOOST_TEST( r.has_error() );
+
+        r &= fi;
+        BOOST_TEST( r.has_error() );
+    }
+
+    {
+        result<Y, E2> r( in_place_value, 1 );
+
+        r &= fy;
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( r->v_, 3 );
+
+        r &= fy2;
+        BOOST_TEST( r.has_error() );
+
+        r &= fy;
+        BOOST_TEST( r.has_error() );
+    }
+
+    {
+        int x1 = 1;
+        result<int&, E2> r( x1 );
+
+        r &= fri;
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( &*r, &*fri( x1 ) );
+
+        r &= fri2;
+        BOOST_TEST( r.has_error() );
+
+        r &= fri;
+        BOOST_TEST( r.has_error() );
+    }
+
+    return boost::report_errors();
+}