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, 2 insertions, 4 deletions

diff --git a/src/wallet/test/spend_tests.cpp b/src/wallet/test/spend_tests.cpp
index 3509bc116f..963c0f838b 100644
--- a/src/wallet/test/spend_tests.cpp
+++ b/src/wallet/test/spend_tests.cpp
@@ -97,13 +97,11 @@ BOOST_FIXTURE_TEST_CASE(wallet_duplicated_preset_inputs_test, TestChain100Setup)
     // so that the recipient's amount is no longer equal to the user's selected target of 299 BTC.
 
     // First case, use 'subtract_fee_from_outputs=true'
-    util::Result<CreatedTransactionResult> res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 
     // Second case, don't use 'subtract_fee_from_outputs'.
     recipients[0].fSubtractFeeFromAmount = false;
-    res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 }
 
 BOOST_AUTO_TEST_SUITE_END()