Remove local implementation of invoke; use Compat

.github/workflows/ci.yml

@@ -59,36 +59,36 @@ jobs:
             install: g++-12
           - toolset: gcc-13
             cxxstd: "11,14,17,20,2b"
-            os: ubuntu-latest
             container: ubuntu:24.04
+            os: ubuntu-latest
             install: g++-13
           - toolset: gcc-14
             cxxstd: "11,14,17,20,2b"
-            os: ubuntu-latest
             container: ubuntu:24.04
+            os: ubuntu-latest
             install: g++-14
           - toolset: gcc-15
             cxxstd: "11,14,17,20,23,2c"
-            os: ubuntu-latest
             container: ubuntu:25.04
+            os: ubuntu-latest
             install: g++-15
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "11,14"
-            os: ubuntu-latest
             container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-3.9
           - toolset: clang
             compiler: clang++-4.0
             cxxstd: "11,14"
-            os: ubuntu-latest
             container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
             cxxstd: "11,14,1z"
-            os: ubuntu-latest
             container: ubuntu:18.04
+            os: ubuntu-latest
             install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0
@@ -99,96 +99,117 @@ jobs:
           - toolset: clang
             compiler: clang++-7
             cxxstd: "11,14,17"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-7
+            install: clang-7 libc++-7-dev libc++abi-7-dev
           - toolset: clang
             compiler: clang++-8
             cxxstd: "11,14,17"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-8
+            install: clang-8 libc++-8-dev libc++abi-8-dev
           - toolset: clang
             compiler: clang++-9
             cxxstd: "11,14,17"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-9
+            install: clang-9 libc++-9-dev libc++abi-9-dev
           - toolset: clang
             compiler: clang++-10
             cxxstd: "11,14,17,2a"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-10
+            install: clang-10 libc++-10-dev libc++abi-10-dev
           - toolset: clang
             compiler: clang++-11
             cxxstd: "11,14,17,2a"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-11
+            install: clang-11 libc++-11-dev libc++abi-11-dev
           - toolset: clang
             compiler: clang++-12
             cxxstd: "11,14,17,20"
+            stdlib: "native,libc++"
             container: ubuntu:20.04
             os: ubuntu-latest
-            install: clang-12
+            install: clang-12 libc++-12-dev libc++abi-12-dev
           - toolset: clang
             compiler: clang++-13
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:22.04
             os: ubuntu-latest
-            install: clang-13
+            install: clang-13 libc++-13-dev libc++abi-13-dev
           - toolset: clang
             compiler: clang++-14
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:22.04
             os: ubuntu-latest
-            install: clang-14
+            install: clang-14 libc++-14-dev libc++abi-14-dev
           - toolset: clang
             compiler: clang++-15
             cxxstd: "11,14,17,20,2b"
             container: ubuntu:22.04
+            stdlib: "native,libc++"
             os: ubuntu-latest
-            install: clang-15
+            install: clang-15 libc++-15-dev libc++abi-15-dev
           - toolset: clang
             compiler: clang++-16
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:24.04
             os: ubuntu-latest
-            install: clang-16
+            install: clang-16 libc++-16-dev libc++abi-16-dev
           - toolset: clang
             compiler: clang++-17
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:24.04
             os: ubuntu-latest
-            install: clang-17
+            install: clang-17 libc++-17-dev libc++abi-17-dev
           - toolset: clang
             compiler: clang++-18
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:24.04
             os: ubuntu-latest
-            install: clang-18
+            install: clang-18 libc++-18-dev libc++abi-18-dev
           - toolset: clang
             compiler: clang++-19
             cxxstd: "11,14,17,20,2b"
+            stdlib: "native,libc++"
             container: ubuntu:24.04
             os: ubuntu-latest
-            install: clang-19
+            install: clang-19 libc++-19-dev libc++abi-19-dev
           - toolset: clang
             compiler: clang++-20
             cxxstd: "11,14,17,20,23,2c"
-            container: ubuntu:25.04
+            stdlib: "native,libc++"
+            container: ubuntu:24.04
             os: ubuntu-latest
-            install: clang-20
+            install: clang-20 libc++-20-dev libc++abi-20-dev
           - toolset: clang
-            cxxstd: "11,14,17,20,2b"
-            os: macos-13
+            compiler: clang++-21
+            cxxstd: "11,14,17,20,23,2c"
+            stdlib: "native,libc++"
+            container: ubuntu:25.10
+            os: ubuntu-latest
+            install: clang-21 libc++-21-dev libc++abi-21-dev
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-14
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-15
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -254,7 +275,7 @@ jobs:
       - name: Run tests
         run: |
           cd ../boost-root
-          ./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} variant=debug,release
+          ./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} variant=debug,release ${{ matrix.stdlib && format('stdlib={0}', matrix.stdlib) }}
 
   windows:
     strategy:
@@ -312,11 +333,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-22.04
-          - os: ubuntu-24.04
-          - os: macos-13
-          - os: macos-14
-          - os: macos-15
+          - os: ubuntu-latest
+          - os: macos-latest
 
     runs-on: ${{matrix.os}}
 
@@ -360,11 +378,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-22.04
-          - os: ubuntu-24.04
-          - os: macos-13
-          - os: macos-14
-          - os: macos-15
+          - os: ubuntu-latest
+          - os: macos-latest
 
     runs-on: ${{matrix.os}}
 
@@ -418,11 +433,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-22.04
-          - os: ubuntu-24.04
-          - os: macos-13
-          - os: macos-14
-          - os: macos-15
+          - os: ubuntu-latest
+          - os: macos-latest
 
     runs-on: ${{matrix.os}}
 

CMakeLists.txt

@@ -2,7 +2,7 @@
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.8...3.20)
+cmake_minimum_required(VERSION 3.8...3.31)
 
 project(boost_system VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
 
@@ -14,6 +14,7 @@ target_include_directories(boost_system INTERFACE include)
 target_link_libraries(boost_system
   INTERFACE
     Boost::assert
+    Boost::compat
     Boost::config
     Boost::throw_exception
     Boost::variant2

doc/system/changes.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2018-2022 Peter Dimov
+Copyright 2018-2025 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -8,6 +8,10 @@ https://www.boost.org/LICENSE_1_0.txt
 # Revision History
 :idprefix:
 
+## Changes in Boost 1.91
+
+* The return type of `operator|(result<T&>, U)` has been changed to non-reference.
+
 ## Changes in Boost 1.89
 
 * The stub compiled library has been removed; System has been header-only since release 1.69.

doc/system/reference.adoc

@@ -2518,8 +2518,8 @@ Returns: ::
 #### operator|
 
 ```
-template<class T, class E, class U> T operator|( result<T, E> const& r, U&& u );
-template<class T, class E, class U> T operator|( result<T, E>&& r, U&& u );
+template<class T, class E, class U> std::decay_t<T> operator|( result<T, E> const& r, U&& u );
+template<class T, class E, class U> std::decay_t<T> operator|( result<T, E>&& r, U&& u );
 ```
 [none]
 * {blank}
@@ -2530,7 +2530,7 @@ Effects: ::
   - If `r.has_value()` is `true`, returns `*r`.
   - Otherwise, returns `u`.
 Remarks: ::
-  Only enabled when `U` is convertible to `T`.
+  Only enabled when `U` is convertible to `std::decay_t<T>`.
 Example: ::
 +
 ```
@@ -2545,6 +2545,7 @@ int get_port()
 ```
 template<class T, class E, class F> T operator|( result<T, E> const& r, F&& f );
 template<class T, class E, class F> T operator|( result<T, E>&& r, F&& f );
+template<class T, class E, class F> std::decay_t<T> operator|( result<T&, E> const& r, F&& f );
 ```
 [none]
 * {blank}
@@ -2556,7 +2557,7 @@ Effects: ::
   - If `r.has_value()` is `true`, returns `*r`.
   - Otherwise, returns `f()`.
 Remarks: ::
-  Only enabled when `f()` is convertible to `T`.
+  Only enabled when `f()` is convertible to `T`, or, when `T` is a reference, to `std::decay_t<T>`.
 Example: ::
 +
 ```

include/boost/system/result.hpp

@@ -1,7 +1,7 @@
 #ifndef BOOST_SYSTEM_RESULT_HPP_INCLUDED
 #define BOOST_SYSTEM_RESULT_HPP_INCLUDED
 
-// Copyright 2017, 2021, 2022 Peter Dimov.
+// Copyright 2017, 2021-2025 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -10,6 +10,7 @@
 #include <boost/system/detail/error_code.hpp>
 #include <boost/system/detail/error_category_impl.hpp>
 #include <boost/variant2/variant.hpp>
+#include <boost/compat/invoke.hpp>
 #include <boost/throw_exception.hpp>
 #include <boost/assert/source_location.hpp>
 #include <boost/assert.hpp>
@@ -919,9 +920,10 @@ template<class T, class E> struct is_result< result<T, E> >: std::true_type {};
 // result | value
 
 template<class T, class E, class U,
-    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+    class En = typename std::enable_if<std::is_convertible<U, typename std::decay<T>::type>::value>::type
 >
-T operator|( result<T, E> const& r, U&& u )
+typename std::decay<T>::type
+operator|( result<T, E> const& r, U&& u )
 {
     if( r )
     {
@@ -934,9 +936,10 @@ T operator|( result<T, E> const& r, U&& u )
 }
 
 template<class T, class E, class U,
-    class En = typename std::enable_if<detail::is_value_convertible_to<U, T>::value>::type
+    class En = typename std::enable_if<std::is_convertible<U, typename std::decay<T>::type>::value>::type
 >
-T operator|( result<T, E>&& r, U&& u )
+typename std::decay<T>::type
+operator|( result<T, E>&& r, U&& u )
 {
     if( r )
     {
@@ -982,6 +985,26 @@ T operator|( result<T, E>&& r, F&& f )
     }
 }
 
+template<class T, class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En = typename std::enable_if<
+        std::is_convertible<U, typename std::decay<T>::type>::value &&
+        !detail::is_value_convertible_to<U, T&>::value
+    >::type
+>
+typename std::decay<T>::type
+operator|( result<T&, E> const& r, F&& f )
+{
+    if( r )
+    {
+        return *r;
+    }
+    else
+    {
+        return std::forward<F>( f )();
+    }
+}
+
 // result | nullary-returning-result
 
 template<class T, class E, class F,
@@ -1108,7 +1131,24 @@ result<T, E>& operator|=( result<T, E>& r, F&& f )
 // result & unary-returning-value
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T const&>() ) ),
+    class U = compat::invoke_result_t<F, T&>,
+    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 )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        return compat::invoke( std::forward<F>( f ), *r );
+    }
+}
+
+template<class T, class E, class F,
+    class U = compat::invoke_result_t<F, T const&>,
     class En1 = typename std::enable_if<!detail::is_result<U>::value>::type,
     class En2 = typename std::enable_if<!std::is_void<U>::value>::type
 >
@@ -1120,12 +1160,12 @@ result<U, E> operator&( result<T, E> const& r, F&& f )
     }
     else
     {
-        return std::forward<F>( f )( *r );
+        return compat::invoke( std::forward<F>( f ), *r );
     }
 }
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class U = typename std::decay< compat::invoke_result_t<F, T> >::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
 >
@@ -1137,12 +1177,29 @@ result<U, E> operator&( result<T, E>&& r, F&& f )
     }
     else
     {
-        return std::forward<F>( f )( *std::move( r ) );
+        return compat::invoke( std::forward<F>( f ), *std::move( r ) );
     }
 }
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T const&>() ) ),
+    class U = compat::invoke_result_t<F, T&>,
+    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 )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        return compat::invoke( std::forward<F>( f ), *std::move( r ) );
+    }
+}
+
+template<class T, class E, class F,
+    class U = compat::invoke_result_t<F, 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 )
@@ -1153,13 +1210,13 @@ result<U, E> operator&( result<T, E> const& r, F&& f )
     }
     else
     {
-        std::forward<F>( f )( *r );
+        compat::invoke( std::forward<F>( f ), *r );
         return {};
     }
 }
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class U = compat::invoke_result_t<F, T>,
     class En = typename std::enable_if<std::is_void<U>::value>::type
 >
 result<U, E> operator&( result<T, E>&& r, F&& f )
@@ -1170,7 +1227,7 @@ result<U, E> operator&( result<T, E>&& r, F&& f )
     }
     else
     {
-        std::forward<F>( f )( *std::move( r ) );
+        compat::invoke( std::forward<F>( f ), *std::move( r ) );
         return {};
     }
 }
@@ -1212,7 +1269,24 @@ result<U, E> operator&( result<void, E> const& r, F&& f )
 // result & unary-returning-result
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T const&>() ) ),
+    class U = typename std::decay< compat::invoke_result_t<F, T&> >::type,
+    class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<std::is_convertible<E, typename U::error_type>::value>::type
+>
+U operator&( result<T, E>& r, F&& f )
+{
+    if( r.has_error() )
+    {
+        return r.error();
+    }
+    else
+    {
+        return compat::invoke( std::forward<F>( f ), *r );
+    }
+}
+
+template<class T, class E, class F,
+    class U = typename std::decay< compat::invoke_result_t<F, T const&> >::type,
     class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
     class En2 = typename std::enable_if<std::is_convertible<E, typename U::error_type>::value>::type
 >
@@ -1224,12 +1298,12 @@ U operator&( result<T, E> const& r, F&& f )
     }
     else
     {
-        return std::forward<F>( f )( *r );
+        return compat::invoke( std::forward<F>( f ), *r );
     }
 }
 
 template<class T, class E, class F,
-    class U = decltype( std::declval<F>()( std::declval<T>() ) ),
+    class U = typename std::decay< compat::invoke_result_t<F, T> >::type,
     class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
     class En2 = typename std::enable_if<std::is_convertible<E, typename U::error_type>::value>::type
 >
@@ -1241,7 +1315,7 @@ U operator&( result<T, E>&& r, F&& f )
     }
     else
     {
-        return std::forward<F>( f )( *std::move( r ) );
+        return compat::invoke( std::forward<F>( f ), *std::move( r ) );
     }
 }
 

test/CMakeLists.txt

@@ -165,8 +165,6 @@ boost_test(TYPE run SOURCES result_value_construct6.cpp)
 boost_test(TYPE run SOURCES result_value_construct7.cpp)
 boost_test(TYPE run SOURCES result_error_construct5.cpp)
 boost_test(TYPE run SOURCES result_or_value.cpp)
-boost_test(TYPE compile-fail SOURCES result_or_value_fail.cpp)
-boost_test(TYPE compile-fail SOURCES result_or_value_fail2.cpp)
 boost_test(TYPE run SOURCES result_or_fn0v.cpp)
 boost_test(TYPE run SOURCES result_or_fn0r.cpp)
 boost_test(TYPE run SOURCES result_and_fn1v.cpp)
@@ -177,3 +175,6 @@ boost_test(TYPE run SOURCES result_in_place_use.cpp)
 boost_test(TYPE run SOURCES result_or_eq_value.cpp)
 boost_test(TYPE run SOURCES result_or_eq_fn0v.cpp)
 boost_test(TYPE run SOURCES result_or_eq_fn0r.cpp)
+
+boost_test(TYPE run SOURCES result_and_mfn1v.cpp)
+boost_test(TYPE run SOURCES result_and_mfn1r.cpp)

test/Jamfile.v2

@@ -198,8 +198,6 @@ run result_value_construct6.cpp ;
 run result_value_construct7.cpp ;
 run result_error_construct5.cpp ;
 run result_or_value.cpp ;
-compile-fail result_or_value_fail.cpp ;
-compile-fail result_or_value_fail2.cpp ;
 run result_or_fn0v.cpp ;
 run result_or_fn0r.cpp ;
 run result_and_fn1v.cpp ;
@@ -210,3 +208,6 @@ run result_in_place_use.cpp ;
 run result_or_eq_value.cpp ;
 run result_or_eq_fn0v.cpp ;
 run result_or_eq_fn0r.cpp ;
+
+run result_and_mfn1v.cpp ;
+run result_and_mfn1r.cpp ;

test/cmake_subdir_test/CMakeLists.txt

@@ -8,6 +8,7 @@ project(cmake_subdir_test LANGUAGES CXX)
 
 add_subdirectory(../.. boostorg/system)
 add_subdirectory(../../../assert boostorg/assert)
+add_subdirectory(../../../compat boostorg/compat)
 add_subdirectory(../../../config boostorg/config)
 add_subdirectory(../../../core boostorg/core)
 add_subdirectory(../../../mp11 boostorg/mp11)

test/result_and_mfn1r.cpp

@@ -0,0 +1,230 @@
+// 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 E
+{
+};
+
+struct E2
+{
+    E2() {}
+    E2( E ) {}
+};
+
+struct X
+{
+    int v_ = 0;
+    mutable int g_called_ = 0;
+
+    X( int v ): v_( v ) {}
+
+    result<int, E2> f() const { return v_; }
+    result<int, E2> f2() const { return E2(); }
+
+    result<int&, E2> g() { return v_; }
+    result<int const&, E2> g2() const { return v_; }
+
+    result<void, E2> h() const { return {}; }
+    result<void, E2> h2() const { return E2(); }
+};
+
+int main()
+{
+    {
+        result<X, E> r( 1 );
+
+        {
+            result<int, E2> r2 = r & &X::f;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+        }
+
+        {
+            result<int, E2> r2 = r & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int&, E2> r2 = r & &X::g;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &r->v_ );
+        }
+
+        {
+            result<int const&, E2> r2 = r & &X::g2;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &r->v_ );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h;
+            BOOST_TEST( r2.has_value() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        result<X, E> const r( 1 );
+
+        {
+            result<int, E2> r2 = r & &X::f;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+        }
+
+        {
+            result<int, E2> r2 = r & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int const&, E2> r2 = r & &X::g2;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &r->v_ );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h;
+            BOOST_TEST( r2.has_value() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        {
+            result<int, E2> r2 = result<X, E>( 1 ) & &X::f;
+            BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+        }
+
+        {
+            result<int, E2> r2 = result<X, E>( 1 ) & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int&, E2> r2 = result<X, E>( 1 ) & &X::g;
+            BOOST_TEST( r2.has_value() );
+        }
+
+        {
+            result<int const&, E2> r2 = result<X, E>( 1 ) & &X::g2;
+            BOOST_TEST( r2.has_value() );
+        }
+
+        {
+            result<void, E2> r2 = result<X, E>( 1 ) & &X::h;
+            BOOST_TEST( r2.has_value() );
+        }
+
+        {
+            result<void, E2> r2 = result<X, E>( 1 ) & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        result<X, E> r( in_place_error );
+
+        {
+            result<int, E2> r2 = r & &X::f;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int, E2> r2 = r & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int&, E2> r2 = r & &X::g;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int const&, E2> r2 = r & &X::g2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        result<X, E> const r( in_place_error );
+
+        {
+            result<int, E2> r2 = r & &X::f;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int, E2> r2 = r & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int const&, E2> r2 = r & &X::g2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = r & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    {
+        {
+            result<int, E2> r2 = result<X, E>( in_place_error ) & &X::f;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int, E2> r2 = result<X, E>( in_place_error ) & &X::f2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int&, E2> r2 = result<X, E>( in_place_error ) & &X::g;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<int const&, E2> r2 = result<X, E>( in_place_error ) & &X::g2;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = result<X, E>( in_place_error ) & &X::h;
+            BOOST_TEST( r2.has_error() );
+        }
+
+        {
+            result<void, E2> r2 = result<X, E>( in_place_error ) & &X::h2;
+            BOOST_TEST( r2.has_error() );
+        }
+    }
+
+    return boost::report_errors();
+}

test/result_and_mfn1v.cpp

@@ -0,0 +1,254 @@
+// 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_ = 0;
+    mutable int g_called_ = 0;
+
+    X( int v ): v_( v ) {}
+
+    int f() const { return v_; }
+
+    void g() const { ++g_called_; }
+
+    int& h() { return v_; }
+    int const& h2() const { return v_; }
+};
+
+struct E
+{
+};
+
+int main()
+{
+    {
+        result<X> r( 1 );
+        result<int> r2 = r & &X::f;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<X> const r( 1 );
+        result<int> r2 = r & &X::f;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<int> r2 = result<X>( 1 ) & &X::f;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<X, E> r( in_place_error );
+        result<int, E> r2 = r & &X::f;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X, E> const r( in_place_error );
+        result<int, E> r2 = r & &X::f;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E> r2 = result<X, E>( in_place_error ) & &X::f;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X> r( 1 );
+        result<int&> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &r->v_ );
+    }
+
+    {
+        result<X> const r( 1 );
+        result<int const&> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &r->v_ );
+    }
+
+    {
+        result<int> r2 = result<X>( 1 ) & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<X, E> r( in_place_error );
+        result<int&, E> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X, E> const r( in_place_error );
+        result<int const&, E> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E> r2 = result<X, E>( in_place_error ) & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        X x( 1 );
+
+        result<X&> r( x );
+        result<int&> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x.v_ );
+    }
+
+    {
+        X const x( 1 );
+
+        result<X const&> r( x );
+        result<int const&> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x.v_ );
+    }
+
+    {
+        X x( 1 );
+
+        result<int&> r2 = result<X&>( x ) & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x.v_ );
+    }
+
+    {
+        X const x( 1 );
+
+        result<int const&> r2 = result<X const&>( x ) & &X::v_;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x.v_ );
+    }
+
+    {
+        result<X&, E> r( in_place_error );
+        result<int&, E> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X const&, E> const r( in_place_error );
+        result<int const&, E> r2 = r & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int&, E> r2 = result<X&, E>( in_place_error ) & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int const&, E> r2 = result<X const&, E>( in_place_error ) & &X::v_;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X> r( 1 );
+        result<void> r2 = r & &X::g;
+
+        BOOST_TEST( r2.has_value() );
+        BOOST_TEST_EQ( r->g_called_, 1 );
+    }
+
+    {
+        result<X> const r( 1 );
+        result<void> r2 = r & &X::g;
+
+        BOOST_TEST( r2.has_value() );
+        BOOST_TEST_EQ( r->g_called_, 1 );
+    }
+
+    {
+        result<void> r2 = result<X>( 1 ) & &X::g;
+
+        BOOST_TEST( r2.has_value() );
+    }
+
+    {
+        result<X, E> r( in_place_error );
+        result<void, E> r2 = r & &X::g;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X, E> const r( in_place_error );
+        result<void, E> r2 = r & &X::g;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<void, E> r2 = result<X, E>( in_place_error ) & &X::g;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X> r( 1 );
+        result<int&> r2 = r & &X::h;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<X> const r( 1 );
+        result<int const&> r2 = r & &X::h2;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<int> r2 = result<X>( 1 ) & &X::h2;
+
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( *r2, 1 );
+    }
+
+    {
+        result<X, E> r( in_place_error );
+        result<int&, E> r2 = r & &X::h;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<X, E> const r( in_place_error );
+        result<int const&, E> r2 = r & &X::h2;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    {
+        result<int, E> r2 = result<X, E>( in_place_error ) & &X::h2;
+
+        BOOST_TEST( r2.has_error() );
+    }
+
+    return boost::report_errors();
+}

test/result_or_fn0r.cpp

@@ -191,120 +191,102 @@ int main()
 
     {
         int x1 = 1;
-        int x3 = 3;
 
         result<int&> r( x1 );
 
-        int& x = r | fri | x3;
+        auto r2 = r | fri;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
         int x1 = 1;
-        int x3 = 3;
 
         result<int&> const r( x1 );
 
-        int& x = r | fri | x3;
+        auto r2 = r | fri;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
         int x1 = 1;
-        int x3 = 3;
 
-        int& x = result<int&>( x1 ) | fri | x3;
+        auto r2 = result<int&>( x1 ) | fri;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
         int x1 = 1;
-        int x3 = 3;
 
         result<int&> r( x1 );
 
-        int& x = r | fri2 | x3;
+        auto r2 = r | fri2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
         int x1 = 1;
-        int x3 = 3;
 
         result<int&> const r( x1 );
 
-        int& x = r | fri2 | x3;
+        auto r2 = r | fri2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
         int x1 = 1;
-        int x3 = 3;
 
-        int& x = result<int&>( x1 ) | fri2 | x3;
+        auto r2 = result<int&>( x1 ) | fri2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &x1 );
     }
 
     {
-        int x3 = 3;
-
         result<int&, E> r( in_place_error );
 
-        int& x = r | fri | x3;
+        auto r2 = r | fri;
 
-        BOOST_TEST_EQ( &x, &*fri() );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &*fri() );
     }
 
     {
-        int x3 = 3;
-
         result<int&, E> const r( in_place_error );
 
-        int& x = r | fri | x3;
+        auto r2 = r | fri;
 
-        BOOST_TEST_EQ( &x, &*fri() );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &*fri() );
     }
 
     {
-        int x3 = 3;
+        auto r2 = result<int&, E>( in_place_error ) | fri;
 
-        int& x = result<int&, E>( in_place_error ) | fri | x3;
-
-        BOOST_TEST_EQ( &x, &*fri() );
+        BOOST_TEST( r2.has_value() ) && BOOST_TEST_EQ( &*r2, &*fri() );
     }
 
     {
-        int x3 = 3;
-
         result<int&, E> r( in_place_error );
 
-        int& x = r | fri2 | x3;
+        auto r2 = r | fri2;
 
-        BOOST_TEST_EQ( &x, &x3 );
+        BOOST_TEST( r2.has_error() );
     }
 
     {
-        int x3 = 3;
-
         result<int&, E> const r( in_place_error );
 
-        int& x = r | fri2 | x3;
+        auto r2 = r | fri2;
 
-        BOOST_TEST_EQ( &x, &x3 );
+        BOOST_TEST( r2.has_error() );
     }
 
     {
-        int x3 = 3;
+        auto r2 = result<int&, E>( in_place_error ) | fri2;
 
-        int& x = result<int&, E>( in_place_error ) | fri2 | x3;
-
-        BOOST_TEST_EQ( &x, &x3 );
+        BOOST_TEST( r2.has_error() );
     }
 
     {

test/result_or_fn0v.cpp

@@ -4,6 +4,8 @@
 
 #include <boost/system/result.hpp>
 #include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
 
 using namespace boost::system;
 
@@ -108,6 +110,71 @@ int main()
         BOOST_TEST_EQ( y.v_, 2 );
     }
 
+    {
+        int x1 = 1;
+
+        result<int&> r( x1 );
+
+        auto&& x2 = r | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, x1 );
+        BOOST_TEST_NE( &x2, &x1 );
+    }
+
+    {
+        int x1 = 1;
+
+        result<int&> const r( x1 );
+
+        auto&& x2 = r | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, x1 );
+        BOOST_TEST_NE( &x2, &x1 );
+    }
+
+    {
+        int x1 = 1;
+
+        auto&& x2 = result<int&>( x1 ) | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, x1 );
+        BOOST_TEST_NE( &x2, &x1 );
+    }
+
+    {
+        result<int&, E> r( in_place_error );
+
+        auto&& x2 = r | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, f() );
+    }
+
+    {
+        result<int&, E> const r( in_place_error );
+
+        auto&& x2 = r | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, f() );
+    }
+
+    {
+        auto&& x2 = result<int&, E>( in_place_error ) | f;
+
+        BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x2)> ));
+
+        BOOST_TEST_EQ( x2, f() );
+    }
+
     {
         int x1 = 1;
 

test/result_or_value.cpp

@@ -4,6 +4,8 @@
 
 #include <boost/system/result.hpp>
 #include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
 
 using namespace boost::system;
 
@@ -110,9 +112,22 @@ int main()
 
         result<int&> r( x1 );
 
-        int& x = r | x2;
+        {
+            auto&& x3 = r | x2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x1 );
+            BOOST_TEST_NE( &x3, &x1 );
+        }
+
+        {
+            auto&& x4 = r | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, x1 );
+        }
     }
 
     {
@@ -121,18 +136,44 @@ int main()
 
         result<int&> const r( x1 );
 
-        int& x = r | x2;
+        {
+            auto&& x3 = r | x2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x1 );
+            BOOST_TEST_NE( &x3, &x1 );
+        }
+
+        {
+            auto&& x4 = r | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, x1 );
+        }
     }
 
     {
         int x1 = 1;
         int x2 = 2;
 
-        int& x = result<int&>( x1 ) | x2;
+        {
+            auto&& x3 = result<int&>( x1 ) | x2;
 
-        BOOST_TEST_EQ( &x, &x1 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x1 );
+            BOOST_TEST_NE( &x3, &x1 );
+        }
+
+        {
+            auto&& x4 = result<int&>( x1 ) | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, x1 );
+        }
     }
 
     {
@@ -140,9 +181,22 @@ int main()
 
         result<int&, E> r( in_place_error );
 
-        int& x = r | x2;
+        {
+            auto&& x3 = r | x2;
 
-        BOOST_TEST_EQ( &x, &x2 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x2 );
+            BOOST_TEST_NE( &x3, &x2 );
+        }
+
+        {
+            auto&& x4 = r | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, 3 );
+        }
     }
 
     {
@@ -150,17 +204,43 @@ int main()
 
         result<int&, E> const r( in_place_error );
 
-        int& x = r | x2;
+        {
+            auto&& x3 = r | x2;
 
-        BOOST_TEST_EQ( &x, &x2 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x2 );
+            BOOST_TEST_NE( &x3, &x2 );
+        }
+
+        {
+            auto&& x4 = r | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, 3 );
+        }
     }
 
     {
         int x2 = 2;
 
-        int& x = result<int&, E>( in_place_error ) | x2;
+        {
+            auto&& x3 = result<int&, E>( in_place_error ) | x2;
 
-        BOOST_TEST_EQ( &x, &x2 );
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype(x3)> ));
+
+            BOOST_TEST_EQ( x3, x2 );
+            BOOST_TEST_NE( &x3, &x2 );
+        }
+
+        {
+            auto&& x4 = result<int&, E>( in_place_error ) | 3;
+
+            BOOST_TEST_TRAIT_FALSE(( std::is_lvalue_reference<decltype( x4 )> ));
+
+            BOOST_TEST_EQ( x4, 3 );
+        }
     }
 
     return boost::report_errors();

test/result_or_value_fail.cpp

@@ -1,14 +0,0 @@
-// 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>
-
-using namespace boost::system;
-
-int main()
-{
-    int x = 1;
-    result<int const&> r( x );
-    r | 2;
-}

test/result_or_value_fail2.cpp

@@ -1,13 +0,0 @@
-// 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>
-
-using namespace boost::system;
-
-int main()
-{
-    int x = 1;
-    result<int const&>( x ) | 2;
-}