refactor: Drop util::Result operator=

`util::Result` objects are aggregates that can hold multiple fields with
different information. Currently Result objects can only hold a success value
of an arbitrary type or a single bilingual_str error message. In followup PR
https://github.com/bitcoin/bitcoin/pull/25722, Result objects may be able to
hold both success and failure values of different types, plus error and warning
messages.

Having a Result::operator= assignment operator that completely erases all
existing Result information before assigning new information is potentially
dangerous in this case. For example, code that looks like it is assigning a
warning value could erase previously-assigned success or failure values.
Conversely, code that looks like it is just assigning a success or failure
value could erase previously assigned error and warning messages.

To prevent potential bugs like this, disable Result::operator= assignment
operator.

It is possible in the future we may want to re-enable operator= in limited
cases (such as when implicit conversions are not used) or add a Replace() or
Reset() method that mimicks default operator= behavior. Followup PR
https://github.com/bitcoin/bitcoin/pull/25722 also adds a Result::Update()
method providing another way to update an existing Result object.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/test/mempool_tests.cpp b/src/test/mempool_tests.cpp
index 217e4a6d22..9f8d434213 100644
--- a/src/test/mempool_tests.cpp
+++ b/src/test/mempool_tests.cpp
@@ -202,9 +202,11 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     tx7.vout[1].scriptPubKey = CScript() << OP_11 << OP_EQUAL;
     tx7.vout[1].nValue = 1 * COIN;
 
-    auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(2000000LL).FromTx(tx7), CTxMemPool::Limits::NoLimits())};
-    BOOST_REQUIRE(ancestors_calculated.has_value());
-    BOOST_CHECK(*ancestors_calculated == setAncestors);
+    {
+        auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(2000000LL).FromTx(tx7), CTxMemPool::Limits::NoLimits())};
+        BOOST_REQUIRE(ancestors_calculated.has_value());
+        BOOST_CHECK(*ancestors_calculated == setAncestors);
+    }
 
     pool.addUnchecked(entry.FromTx(tx7), setAncestors);
     BOOST_CHECK_EQUAL(pool.size(), 7U);
@@ -260,9 +262,11 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     tx10.vout[0].scriptPubKey = CScript() << OP_11 << OP_EQUAL;
     tx10.vout[0].nValue = 10 * COIN;
 
-    ancestors_calculated = pool.CalculateMemPoolAncestors(entry.Fee(200000LL).Time(NodeSeconds{4s}).FromTx(tx10), CTxMemPool::Limits::NoLimits());
-    BOOST_REQUIRE(ancestors_calculated);
-    BOOST_CHECK(*ancestors_calculated == setAncestors);
+    {
+        auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(200000LL).Time(NodeSeconds{4s}).FromTx(tx10), CTxMemPool::Limits::NoLimits())};
+        BOOST_REQUIRE(ancestors_calculated);
+        BOOST_CHECK(*ancestors_calculated == setAncestors);
+    }
 
     pool.addUnchecked(entry.FromTx(tx10), setAncestors);