refactor: Replace BResult with util::Result

Rename `BResult` class to `util::Result` and update the class interface to be
more compatible with `std::optional` and with a full-featured result class
implemented in https://github.com/bitcoin/bitcoin/pull/25665. Motivation for
this change is to update existing `BResult` usages now so they don't have to
change later when more features are added in #25665.

This change makes the following improvements originally implemented in #25665:

- More explicit API. Drops potentially misleading `BResult` constructor that
  treats any bilingual string argument as an error. Adds `util::Error`
  constructor so it is never ambiguous when a result is being assigned an error
  or non-error value.

- Better type compatibility. Supports `util::Result<bilingual_str>` return
  values to hold translated messages which are not errors.

- More standard and consistent API. `util::Result` supports most of the same
  operators and methods as `std::optional`. `BResult` had a less familiar
  interface with `HasRes`/`GetObj`/`ReleaseObj` methods. The Result/Res/Obj
  naming was also not internally consistent.

- Better code organization. Puts `src/util/` code in the `util::` namespace so
  naming reflects code organization and it is obvious where the class is coming
  from. Drops "B" from name because it is undocumented what it stands for
  (bilingual?)

- Has unit tests.

2 files changed, 98 insertions, 4 deletions

diff --git a/src/test/result_tests.cpp b/src/test/result_tests.cpp
new file mode 100644
index 0000000000..6a23a7b895
--- /dev/null
+++ b/src/test/result_tests.cpp
@@ -0,0 +1,96 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/result.h>
+
+#include <boost/test/unit_test.hpp>
+
+inline bool operator==(const bilingual_str& a, const bilingual_str& b)
+{
+    return a.original == b.original && a.translated == b.translated;
+}
+
+inline std::ostream& operator<<(std::ostream& os, const bilingual_str& s)
+{
+    return os << "bilingual_str('" << s.original << "' , '" << s.translated << "')";
+}
+
+BOOST_AUTO_TEST_SUITE(result_tests)
+
+struct NoCopy {
+    NoCopy(int n) : m_n{std::make_unique<int>(n)} {}
+    std::unique_ptr<int> m_n;
+};
+
+bool operator==(const NoCopy& a, const NoCopy& b)
+{
+    return *a.m_n == *b.m_n;
+}
+
+std::ostream& operator<<(std::ostream& os, const NoCopy& o)
+{
+    return os << "NoCopy(" << *o.m_n << ")";
+}
+
+util::Result<int> IntFn(int i, bool success)
+{
+    if (success) return i;
+    return util::Error{Untranslated(strprintf("int %i error.", i))};
+}
+
+util::Result<bilingual_str> StrFn(bilingual_str s, bool success)
+{
+    if (success) return s;
+    return util::Error{strprintf(Untranslated("str %s error."), s.original)};
+}
+
+util::Result<NoCopy> NoCopyFn(int i, bool success)
+{
+    if (success) return {i};
+    return util::Error{Untranslated(strprintf("nocopy %i error.", i))};
+}
+
+template <typename T>
+void ExpectResult(const util::Result<T>& result, bool success, const bilingual_str& str)
+{
+    BOOST_CHECK_EQUAL(bool(result), success);
+    BOOST_CHECK_EQUAL(util::ErrorString(result), str);
+}
+
+template <typename T, typename... Args>
+void ExpectSuccess(const util::Result<T>& result, const bilingual_str& str, Args&&... args)
+{
+    ExpectResult(result, true, str);
+    BOOST_CHECK_EQUAL(result.has_value(), true);
+    BOOST_CHECK_EQUAL(result.value(), T{std::forward<Args>(args)...});
+    BOOST_CHECK_EQUAL(&result.value(), &*result);
+}
+
+template <typename T, typename... Args>
+void ExpectFail(const util::Result<T>& result, const bilingual_str& str)
+{
+    ExpectResult(result, false, str);
+}
+
+BOOST_AUTO_TEST_CASE(check_returned)
+{
+    ExpectSuccess(IntFn(5, true), {}, 5);
+    ExpectFail(IntFn(5, false), Untranslated("int 5 error."));
+    ExpectSuccess(NoCopyFn(5, true), {}, 5);
+    ExpectFail(NoCopyFn(5, false), Untranslated("nocopy 5 error."));
+    ExpectSuccess(StrFn(Untranslated("S"), true), {}, Untranslated("S"));
+    ExpectFail(StrFn(Untranslated("S"), false), Untranslated("str S error."));
+}
+
+BOOST_AUTO_TEST_CASE(check_value_or)
+{
+    BOOST_CHECK_EQUAL(IntFn(10, true).value_or(20), 10);
+    BOOST_CHECK_EQUAL(IntFn(10, false).value_or(20), 20);
+    BOOST_CHECK_EQUAL(NoCopyFn(10, true).value_or(20), 10);
+    BOOST_CHECK_EQUAL(NoCopyFn(10, false).value_or(20), 20);
+    BOOST_CHECK_EQUAL(StrFn(Untranslated("A"), true).value_or(Untranslated("B")), Untranslated("A"));
+    BOOST_CHECK_EQUAL(StrFn(Untranslated("A"), false).value_or(Untranslated("B")), Untranslated("B"));
+}
+
+BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/util/wallet.cpp b/src/test/util/wallet.cpp
index 7a00ac9e1f..b54774cbb9 100644
--- a/src/test/util/wallet.cpp
+++ b/src/test/util/wallet.cpp
@@ -8,6 +8,7 @@
 #include <outputtype.h>
 #include <script/standard.h>
 #ifdef ENABLE_WALLET
+#include <util/check.h>
 #include <util/translation.h>
 #include <wallet/wallet.h>
 #endif
@@ -20,10 +21,7 @@ const std::string ADDRESS_BCRT1_UNSPENDABLE = "bcrt1qqqqqqqqqqqqqqqqqqqqqqqqqqqq
 std::string getnewaddress(CWallet& w)
 {
     constexpr auto output_type = OutputType::BECH32;
-    auto op_dest = w.GetNewDestination(output_type, "");
-    assert(op_dest.HasRes());
-
-    return EncodeDestination(op_dest.GetObj());
+    return EncodeDestination(*Assert(w.GetNewDestination(output_type, "")));
 }
 
 #endif // ENABLE_WALLET