util: Add Unexpected::error()

This is not needed, but a bit closer to the std lib.

src/test/util_expected_tests.cpp

@@ -7,6 +7,11 @@
 
 #include <boost/test/unit_test.hpp>
 
+#include <memory>
+#include <string>
+#include <utility>
+
+
 using namespace util;
 
 BOOST_AUTO_TEST_SUITE(util_expected_tests)
@@ -65,4 +70,17 @@ BOOST_AUTO_TEST_CASE(expected_error)
     BOOST_CHECK_EQUAL(read.error(), "fail1");
 }
 
+BOOST_AUTO_TEST_CASE(unexpected_error_accessors)
+{
+    Unexpected u{std::make_unique<int>(-1)};
+    BOOST_CHECK_EQUAL(*u.error(), -1);
+
+    *u.error() -= 1;
+    const auto& read{u};
+    BOOST_CHECK_EQUAL(*read.error(), -2);
+
+    const auto moved{std::move(u).error()};
+    BOOST_CHECK_EQUAL(*moved, -2);
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/util/expected.h

@@ -20,8 +20,14 @@ template <class E>
 class Unexpected
 {
 public:
-    constexpr explicit Unexpected(E e) : err(std::move(e)) {}
+    constexpr explicit Unexpected(E e) : m_error(std::move(e)) {}
-    E err;
+
+    constexpr const E& error() const& noexcept LIFETIMEBOUND { return m_error; }
+    constexpr E& error() & noexcept LIFETIMEBOUND { return m_error; }
+    constexpr E&& error() && noexcept LIFETIMEBOUND { return std::move(m_error); }
+
+private:
+    E m_error;
 };
 
 /// The util::Expected class provides a standard way for low-level functions to
@@ -40,7 +46,7 @@ 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)} {}
     template <class Err>
-    constexpr Expected(Unexpected<Err> u) : m_data{std::in_place_index_t<1>{}, std::move(u.err)}
+    constexpr Expected(Unexpected<Err> u) : m_data{std::in_place_index_t<1>{}, std::move(u).error()}
     {
     }