Add util::Expected (std::expected)

src/test/CMakeLists.txt

@@ -119,6 +119,7 @@ add_executable(test_bitcoin
   txvalidationcache_tests.cpp
   uint256_tests.cpp
   util_check_tests.cpp
+  util_expected_tests.cpp
   util_string_tests.cpp
   util_tests.cpp
   util_threadnames_tests.cpp

src/test/util_expected_tests.cpp

@@ -0,0 +1,68 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit.
+
+#include <test/util/setup_common.h>
+#include <util/expected.h>
+
+#include <boost/test/unit_test.hpp>
+
+using namespace util;
+
+BOOST_AUTO_TEST_SUITE(util_expected_tests)
+
+BOOST_AUTO_TEST_CASE(expected_value)
+{
+    struct Obj {
+        int x;
+    };
+    Expected<Obj, int> e{};
+    BOOST_CHECK(e.value().x == 0);
+
+    e = Obj{42};
+
+    BOOST_CHECK(e.has_value());
+    BOOST_CHECK(static_cast<bool>(e));
+    BOOST_CHECK(e.value().x == 42);
+    BOOST_CHECK((*e).x == 42);
+    BOOST_CHECK(e->x == 42);
+
+    // modify value
+    e.value().x += 1;
+    (*e).x += 1;
+    e->x += 1;
+
+    const auto& read{e};
+    BOOST_CHECK(read.value().x == 45);
+    BOOST_CHECK((*read).x == 45);
+    BOOST_CHECK(read->x == 45);
+}
+
+BOOST_AUTO_TEST_CASE(expected_value_or)
+{
+    Expected<std::unique_ptr<int>, int> no_copy{std::make_unique<int>(1)};
+    const int one{*std::move(no_copy).value_or(std::make_unique<int>(2))};
+    BOOST_CHECK_EQUAL(one, 1);
+
+    const Expected<std::string, int> const_val{Unexpected{-1}};
+    BOOST_CHECK_EQUAL(const_val.value_or("fallback"), "fallback");
+}
+
+BOOST_AUTO_TEST_CASE(expected_error)
+{
+    Expected<void, std::string> e{};
+    BOOST_CHECK(e.has_value());
+
+    e = Unexpected{"fail"};
+    BOOST_CHECK(!e.has_value());
+    BOOST_CHECK(!static_cast<bool>(e));
+    BOOST_CHECK(e.error() == "fail");
+
+    // modify error
+    e.error() += "1";
+
+    const auto& read{e};
+    BOOST_CHECK(read.error() == "fail1");
+}
+
+BOOST_AUTO_TEST_SUITE_END()