util: Add Expected::swap()

src/test/util_expected_tests.cpp

@@ -122,4 +122,13 @@ BOOST_AUTO_TEST_CASE(unexpected_error_accessors)
     BOOST_CHECK_EQUAL(*moved, -2);
 }
 
+BOOST_AUTO_TEST_CASE(expected_swap)
+{
+    Expected<const char*, std::unique_ptr<int>> a{Unexpected{std::make_unique<int>(-1)}};
+    Expected<const char*, std::unique_ptr<int>> b{"good"};
+    a.swap(b);
+    BOOST_CHECK_EQUAL(a.value(), "good");
+    BOOST_CHECK_EQUAL(*b.error(), -1);
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/util/expected.h

@@ -88,6 +88,8 @@ public:
     constexpr E& error() & noexcept LIFETIMEBOUND { return *Assert(std::get_if<1>(&m_data)); }
     constexpr E&& error() && noexcept LIFETIMEBOUND { return std::move(error()); }
 
+    constexpr void swap(Expected& other) noexcept { m_data.swap(other.m_data); }
+
     constexpr T& operator*() & noexcept LIFETIMEBOUND { return value(); }
     constexpr const T& operator*() const& noexcept LIFETIMEBOUND { return value(); }
     constexpr T&& operator*() && noexcept LIFETIMEBOUND { return std::move(value()); }