gui: Do not translate InitWarning messages in debug.log #18922

        if (!warnings.empty()) chain.initWarning(Join(warnings, Untranslated("\n"));

Updated.

I think it would be safer to not allow this and force the caller to go through Untranslated

Updated.

auto Join(const std::vector<T>& list, const U& separator, UnaryOp unary_op)

I think this requirement can be lifted (here and below)

Updated.

Please keep the explicit overload for std::string, if you want you can extend it with a comment why it exists.

Also, I like the idea of deriving the return type from the unary operator

diff --git a/src/util/string.h b/src/util/string.h
index 81d4c186e5..e7651c3e7d 100644
--- a/src/util/string.h
+++ b/src/util/string.h
@@ -31,9 +31,10 @@ NODISCARD inline std::string TrimString(const std::string& str, const std::strin
  * [@param](/bitcoin-bitcoin/contributor/param/) unary_op   Apply this operator to each item in the list
  */
 template <typename T, typename BaseType, typename UnaryOp>
-BaseType Join(const std::vector<T>& list, const BaseType& separator, UnaryOp unary_op)
+auto Join(const std::vector<T>& list, const BaseType& separator, UnaryOp unary_op)
+    -> decltype(unary_op(list.at(0)))
 {
-    BaseType ret;
+    decltype(unary_op(list.at(0))) ret;
     for (size_t i = 0; i < list.size(); ++i) {
         if (i > 0) ret += separator;
         ret += unary_op(list.at(i));
@@ -47,6 +48,12 @@ T Join(const std::vector<T>& list, const T& separator)
     return Join(list, separator, [](const T& i) { return i; });
 }
 
+inline std::string Join(const std::vector<std::string>& list, const std::string& separator)
+{
+    // explicit overload needed for c_str, which would otherwise cause a substitution failure in the template above
+    return Join<std::string>(list, separator);
+}
+
 /**
  * Check if a string does not contain any embedded NUL (\0) characters
  */

Updated.

Why are all these unrelated changes needed. This diff compiles fine for me on top:

diff --git a/src/rpc/util.cpp b/src/rpc/util.cpp
index 597da6e7fb..860fa198d5 100644
--- a/src/rpc/util.cpp
+++ b/src/rpc/util.cpp
@@ -751,7 +751,7 @@ std::string RPCArg::ToString(const bool oneline) const
     }
     case Type::OBJ:
     case Type::OBJ_USER_KEYS: {
-        const std::string res = Join(m_inner, std::string(","), [&](const RPCArg& i) { return i.ToStringObj(oneline); });
+        const std::string res = Join(m_inner, ",", [&](const RPCArg& i) { return i.ToStringObj(oneline); });
         if (m_type == Type::OBJ) {
             return "{" + res + "}";
         } else {
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index f71e7efa2b..45b7fd4932 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -141,15 +141,15 @@ BOOST_AUTO_TEST_CASE(util_HexStr)
 BOOST_AUTO_TEST_CASE(util_Join)
 {
     // Normal version
-    BOOST_CHECK_EQUAL(Join({}, std::string(", ")), "");
-    BOOST_CHECK_EQUAL(Join({"foo"}, std::string(", ")), "foo");
-    BOOST_CHECK_EQUAL(Join({"foo", "bar"}, std::string(", ")), "foo, bar");
+    BOOST_CHECK_EQUAL(Join({}, ", "), "");
+    BOOST_CHECK_EQUAL(Join({"foo"}, ", "), "foo");
+    BOOST_CHECK_EQUAL(Join({"foo", "bar"}, ", "), "foo, bar");
 
     // Version with unary operator
     const auto op_upper = [](const std::string& s) { return ToUpper(s); };
-    BOOST_CHECK_EQUAL(Join<std::string>({}, std::string(", "), op_upper), "");
-    BOOST_CHECK_EQUAL(Join<std::string>({"foo"}, std::string(", "), op_upper), "FOO");
-    BOOST_CHECK_EQUAL(Join<std::string>({"foo", "bar"}, std::string(", "), op_upper), "FOO, BAR");
+    BOOST_CHECK_EQUAL(Join<std::string>({}, ", ", op_upper), "");
+    BOOST_CHECK_EQUAL(Join<std::string>({"foo"}, ", ", op_upper), "FOO");
+    BOOST_CHECK_EQUAL(Join<std::string>({"foo", "bar"}, ", ", op_upper), "FOO, BAR");
 }
 
 BOOST_AUTO_TEST_CASE(util_FormatParseISO8601DateTime)
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 0534befd3d..dda00f1fe7 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2597,7 +2597,7 @@ static UniValue loadwallet(const JSONRPCRequest& request)
 
     UniValue obj(UniValue::VOBJ);
     obj.pushKV("name", wallet->GetName());
-    obj.pushKV("warning", Join(warnings, std::string("\n"), OpOriginal));
+    obj.pushKV("warning", Join(warnings, "\n", OpOriginal));
 
     return obj;
 }
@@ -2737,7 +2737,7 @@ static UniValue createwallet(const JSONRPCRequest& request)
 
     UniValue obj(UniValue::VOBJ);
     obj.pushKV("name", wallet->GetName());
-    obj.pushKV("warning", Join(warnings, std::string("\n"), OpOriginal));
+    obj.pushKV("warning", Join(warnings, "\n", OpOriginal));
 
     return obj;
 }

You are right. Those are remnants from the previous version of this branch.

Fixed.

Why is this one needed? (Sorry I missed it in my last review comment)

Thank you. Fixed.