util: Add Expected<void, E> specialization

This is not needed, but a bit closer to the std lib, because
std::monostate is no longer leaked through ValueType from the value()
method.

src/test/util_expected_tests.cpp

@@ -66,6 +66,8 @@ BOOST_AUTO_TEST_CASE(expected_error)
 {
     Expected<void, std::string> e{};
     BOOST_CHECK(e.has_value());
+    [&]() -> void { return e.value(); }(); // check value returns void and does not throw
+    [&]() -> void { return *e; }();
 
     e = Unexpected{"fail"};
     BOOST_CHECK(!e.has_value());

src/util/expected.h

@@ -6,10 +6,10 @@
 #define BITCOIN_UTIL_EXPECTED_H
 
 #include <attributes.h>
+#include <util/check.h>
 
 #include <cassert>
 #include <exception>
-#include <type_traits>
 #include <utility>
 #include <variant>
 
@@ -44,28 +44,27 @@ template <class T, class E>
 class Expected
 {
 private:
-    using ValueType = std::conditional_t<std::is_same_v<T, void>, std::monostate, T>;
-    std::variant<ValueType, E> m_data;
+    std::variant<T, E> m_data;
 
 public:
-    constexpr Expected() : m_data{std::in_place_index_t<0>{}, ValueType{}} {}
-    constexpr Expected(ValueType v) : m_data{std::in_place_index_t<0>{}, std::move(v)} {}
+    constexpr Expected() : m_data{std::in_place_index<0>, T{}} {}
+    constexpr Expected(T v) : m_data{std::in_place_index<0>, std::move(v)} {}
     template <class Err>
-    constexpr Expected(Unexpected<Err> u) : m_data{std::in_place_index_t<1>{}, std::move(u).error()}
+    constexpr Expected(Unexpected<Err> u) : m_data{std::in_place_index<1>, std::move(u).error()}
     {
     }
 
     constexpr bool has_value() const noexcept { return m_data.index() == 0; }
     constexpr explicit operator bool() const noexcept { return has_value(); }
 
-    constexpr const ValueType& value() const LIFETIMEBOUND
+    constexpr const T& value() const LIFETIMEBOUND
     {
         if (!has_value()) {
             throw BadExpectedAccess{};
         }
         return std::get<0>(m_data);
     }
-    constexpr ValueType& value() LIFETIMEBOUND
+    constexpr T& value() LIFETIMEBOUND
     {
         if (!has_value()) {
             throw BadExpectedAccess{};
@@ -74,12 +73,12 @@ public:
     }
 
     template <class U>
-    ValueType value_or(U&& default_value) const&
+    T value_or(U&& default_value) const&
     {
         return has_value() ? value() : std::forward<U>(default_value);
     }
     template <class U>
-    ValueType value_or(U&& default_value) &&
+    T value_or(U&& default_value) &&
     {
         return has_value() ? std::move(value()) : std::forward<U>(default_value);
     }
@@ -95,11 +94,40 @@ public:
         return std::get<1>(m_data);
     }
 
-    constexpr ValueType& operator*() noexcept LIFETIMEBOUND { return value(); }
-    constexpr const ValueType& operator*() const noexcept LIFETIMEBOUND { return value(); }
+    constexpr T& operator*() noexcept LIFETIMEBOUND { return value(); }
+    constexpr const T& operator*() const noexcept LIFETIMEBOUND { return value(); }
 
-    constexpr ValueType* operator->() noexcept LIFETIMEBOUND { return &value(); }
-    constexpr const ValueType* operator->() const noexcept LIFETIMEBOUND { return &value(); }
+    constexpr T* operator->() noexcept LIFETIMEBOUND { return &value(); }
+    constexpr const T* operator->() const noexcept LIFETIMEBOUND { return &value(); }
+};
+
+template <class E>
+class Expected<void, E>
+{
+private:
+    std::variant<std::monostate, E> m_data;
+
+public:
+    constexpr Expected() : m_data{std::in_place_index<0>, std::monostate{}} {}
+    template <class Err>
+    constexpr Expected(Unexpected<Err> u) : m_data{std::in_place_index<1>, std::move(u).error()}
+    {
+    }
+
+    constexpr bool has_value() const noexcept { return m_data.index() == 0; }
+    constexpr explicit operator bool() const noexcept { return has_value(); }
+
+    constexpr void operator*() const noexcept { return value(); }
+    constexpr void value() const
+    {
+        if (!has_value()) {
+            throw BadExpectedAccess{};
+        }
+    }
+
+    constexpr const E& error() const& noexcept LIFETIMEBOUND { return *Assert(std::get_if<1>(&m_data)); }
+    constexpr E& error() & noexcept LIFETIMEBOUND { return *Assert(std::get_if<1>(&m_data)); }
+    constexpr E&& error() && noexcept LIFETIMEBOUND { return std::move(error()); }
 };
 
 } // namespace util