From a9d88987926853d162fa7201a43b5098be589fd3 Mon Sep 17 00:00:00 2001
From: seelabs <scott.determan@yahoo.com>
Date: Wed, 19 Oct 2022 11:48:08 -0400
Subject: [PATCH 1/2] Improve move semantics in Expected:

This patch unconditionally moves an `Unexpected<U>` value parameter as
long as `U` is not a reference. If `U` is a reference the code should
not compile. An error type that holds a reference is a strange use-case,
and an overload is not provided. If it is required in the future it can
be added.

The `Expected(U r)` overload should take a forwarding ref.
---
 src/ripple/basics/Expected.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/ripple/basics/Expected.h b/src/ripple/basics/Expected.h
index 09d2bdc5042..a53200a35f6 100644
--- a/src/ripple/basics/Expected.h
+++ b/src/ripple/basics/Expected.h
@@ -135,14 +135,15 @@ class [[nodiscard]] Expected
     template <
         typename U,
         typename = std::enable_if_t<std::is_convertible_v<U, T>>>
-    constexpr Expected(U r) : Base(T{std::forward<U>(r)})
+    constexpr Expected(U && r) : Base(T{std::forward<U>(r)})
     {
     }
 
     template <
         typename U,
-        typename = std::enable_if_t<std::is_convertible_v<U, E>>>
-    constexpr Expected(Unexpected<U> e) : Base(E{std::forward<U>(e.value())})
+        typename = std::enable_if_t<
+            std::is_convertible_v<U, E> && !std::is_reference_v<U>>>
+    constexpr Expected(Unexpected<U> e) : Base(E{std::move(e.value())})
     {
     }
 
@@ -217,8 +218,9 @@ class [[nodiscard]] Expected<void, E>
 
     template <
         typename U,
-        typename = std::enable_if_t<std::is_convertible_v<U, E>>>
-    constexpr Expected(Unexpected<U> e) : Base(E{std::forward<U>(e.value())})
+        typename = std::enable_if_t<
+            std::is_convertible_v<U, E> && !std::is_reference_v<U>>>
+    constexpr Expected(Unexpected<U> e) : Base(E{std::move(e.value())})
     {
     }
 

From 4b288aa950fdb9ad013df35f045431d13170fc81 Mon Sep 17 00:00:00 2001
From: seelabs <scott.determan@yahoo.com>
Date: Wed, 19 Oct 2022 15:40:03 -0400
Subject: [PATCH 2/2] Replace enable_if with concepts in Expected

---
 src/ripple/basics/Expected.h | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/src/ripple/basics/Expected.h b/src/ripple/basics/Expected.h
index a53200a35f6..8dc368eefd7 100644
--- a/src/ripple/basics/Expected.h
+++ b/src/ripple/basics/Expected.h
@@ -21,7 +21,10 @@
 #define RIPPLE_BASICS_EXPECTED_H_INCLUDED
 
 #include <ripple/basics/contract.h>
+
 #include <boost/outcome.hpp>
+
+#include <concepts>
 #include <stdexcept>
 #include <type_traits>
 
@@ -132,18 +135,16 @@ class [[nodiscard]] Expected
     using Base = boost::outcome_v2::result<T, E, detail::throw_policy>;
 
 public:
-    template <
-        typename U,
-        typename = std::enable_if_t<std::is_convertible_v<U, T>>>
-    constexpr Expected(U && r) : Base(T{std::forward<U>(r)})
+    template <typename U>
+    requires std::convertible_to<U, T> constexpr Expected(U && r)
+        : Base(T{std::forward<U>(r)})
     {
     }
 
-    template <
-        typename U,
-        typename = std::enable_if_t<
-            std::is_convertible_v<U, E> && !std::is_reference_v<U>>>
-    constexpr Expected(Unexpected<U> e) : Base(E{std::move(e.value())})
+    template <typename U>
+        requires std::convertible_to<U, E> &&
+        (!std::is_reference_v<U>)constexpr Expected(Unexpected<U> e)
+        : Base(E{std::move(e.value())})
     {
     }
 
@@ -216,11 +217,10 @@ class [[nodiscard]] Expected<void, E>
     {
     }
 
-    template <
-        typename U,
-        typename = std::enable_if_t<
-            std::is_convertible_v<U, E> && !std::is_reference_v<U>>>
-    constexpr Expected(Unexpected<U> e) : Base(E{std::move(e.value())})
+    template <typename U>
+        requires std::convertible_to<U, E> &&
+        (!std::is_reference_v<U>)constexpr Expected(Unexpected<U> e)
+        : Base(E{std::move(e.value())})
     {
     }