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, 6 insertions, 5 deletions

diff --git a/src/qt/addresstablemodel.cpp b/src/qt/addresstablemodel.cpp
index c52ef7cd67..efdc3966d1 100644
--- a/src/qt/addresstablemodel.cpp
+++ b/src/qt/addresstablemodel.cpp
@@ -369,21 +369,22 @@ QString AddressTableModel::addRow(const QString &type, const QString &label, con
     else if(type == Receive)
     {
         // Generate a new address to associate with given label
-        auto op_dest = walletModel->wallet().getNewDestination(address_type, strLabel);
-        if (!op_dest) {
+        if (auto dest{walletModel->wallet().getNewDestination(address_type, strLabel)}) {
+            strAddress = EncodeDestination(*dest);
+        } else {
             WalletModel::UnlockContext ctx(walletModel->requestUnlock());
             if (!ctx.isValid()) {
                 // Unlock wallet failed or was cancelled
                 editStatus = WALLET_UNLOCK_FAILURE;
                 return QString();
             }
-            op_dest = walletModel->wallet().getNewDestination(address_type, strLabel);
-            if (!op_dest) {
+            if (auto dest_retry{walletModel->wallet().getNewDestination(address_type, strLabel)}) {
+                strAddress = EncodeDestination(*dest_retry);
+            } else {
                 editStatus = KEY_GENERATION_FAILURE;
                 return QString();
             }
         }
-        strAddress = EncodeDestination(*op_dest);
     }
     else
     {