From f0b27c58267235732f663b9f9b6d4c9d16fc1840 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sat, 21 Jan 2023 20:09:53 +0200
Subject: [PATCH] Enable implicit construction when the alternative is
 explicitly constructible from the argument. Fixes #103.

---
 include/boost/system/result.hpp |  4 ++--
 test/result_error_construct.cpp | 10 ++++++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/boost/system/result.hpp b/include/boost/system/result.hpp
index fb2f9bd..33eec39 100644
--- a/include/boost/system/result.hpp
+++ b/include/boost/system/result.hpp
@@ -122,7 +122,7 @@ public:
     template<class A = T, typename std::enable_if<
         std::is_convertible<A, T>::value &&
         !(detail::is_errc_t<A>::value && std::is_arithmetic<T>::value) &&
-        !std::is_constructible<E, A>::value, int>::type = 0>
+        !std::is_convertible<A, E>::value, int>::type = 0>
     constexpr result( A&& a )
         noexcept( std::is_nothrow_constructible<T, A>::value )
         : v_( in_place_value, std::forward<A>(a) )
@@ -132,7 +132,7 @@ public:
     // implicit, error
     template<class A = E, class = void, typename std::enable_if<
         std::is_convertible<A, E>::value &&
-        !std::is_constructible<T, A>::value, int>::type = 0>
+        !std::is_convertible<A, T>::value, int>::type = 0>
     constexpr result( A&& a )
         noexcept( std::is_nothrow_constructible<E, A>::value )
         : v_( in_place_error, std::forward<A>(a) )
diff --git a/test/result_error_construct.cpp b/test/result_error_construct.cpp
index 8cee5a9..af9de73 100644
--- a/test/result_error_construct.cpp
+++ b/test/result_error_construct.cpp
@@ -146,8 +146,14 @@ int main()
         BOOST_TEST_TRAIT_TRUE((std::is_constructible<result<std::string, X>, int>));
         BOOST_TEST_TRAIT_FALSE((std::is_convertible<int, result<std::string, X>>));
 
-        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int, X>, int>));
-        BOOST_TEST_TRAIT_FALSE((std::is_convertible<int, result<int, X>>));
+        // We'd like this to be false due to the ambiguity caused by X having
+        // an explicit constructor taking an int, which should be viable in this
+        // context, but the implicit constructor is enabled, and there's no way to
+        // disallow it
+        //
+        // BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int, X>, int>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_convertible<int, result<int, X>>));
     }
 
     {