Merge bitcoin#20736: rpc: Replace boost::variant with std::variant for RPCArg.m_fallback

fa749fb rpc: Replace boost::variant with std::variant for RPCArg.m_fallback (MarcoFalke)

Pull request description:

  Now that we can use std::variant from the vanilla standard library, drop the third-party boost variant dependency.

  Patch is split out from bitcoin#20480. A step-by-step replacement is possible because we don't have our own `Variant` wrapper and the source code specifies `boost::variant` explicitly.
  I think a step-by-step replacement should be preferred, because it simplifies review.

ACKs for top commit:
  fjahr:
    re-ACK fa749fb
  Sjors:
    re-ACK fa749fb

Tree-SHA512: 5e3c12b7d535f73065b4afa8df0a488f78fb25d2234f5ecbf740e624db03a34c35fea100eb7d37e84741721310e6450b7fb4296a2207a7ed1fa24485b3650981

Author: MarcoFalke

src/rpc/util.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2020 The Bitcoin Core developers
+// Copyright (c) 2017-2021 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -562,10 +562,10 @@ std::string RPCArg::GetName() const
 
 bool RPCArg::IsOptional() const
 {
-    if (m_fallback.which() == 1) {
+    if (m_fallback.index() == 1) {
         return true;
     } else {
-        return RPCArg::Optional::NO != boost::get<RPCArg::Optional>(m_fallback);
+        return RPCArg::Optional::NO != std::get<RPCArg::Optional>(m_fallback);
     }
 }
 
@@ -609,10 +609,10 @@ std::string RPCArg::ToDescriptionString() const
         }
         } // no default case, so the compiler can warn about missing cases
     }
-    if (m_fallback.which() == 1) {
-        ret += ", optional, default=" + boost::get<std::string>(m_fallback);
+    if (m_fallback.index() == 1) {
+        ret += ", optional, default=" + std::get<std::string>(m_fallback);
     } else {
-        switch (boost::get<RPCArg::Optional>(m_fallback)) {
+        switch (std::get<RPCArg::Optional>(m_fallback)) {
         case RPCArg::Optional::OMITTED: {
             // nothing to do. Element is treated as if not present and has no default value
             break;

src/rpc/util.h

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2020 The Bitcoin Core developers
+// Copyright (c) 2017-2021 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -19,10 +19,9 @@
 #include <util/check.h>
 
 #include <string>
+#include <variant>
 #include <vector>
 
-#include <boost/variant.hpp>
-
 /**
  * String used to describe UNIX epoch time in documentation, factored out to a
  * constant for consistency.
@@ -144,7 +143,7 @@ struct RPCArg {
          */
         OMITTED,
     };
-    using Fallback = boost::variant<Optional, /* default value for optional args */ std::string>;
+    using Fallback = std::variant<Optional, /* default value for optional args */ std::string>;
     const std::string m_names; //!< The name of the arg (can be empty for inner args, can contain multiple aliases separated by | for named request arguments)
     const Type m_type;
     const bool m_hidden;

test/sanitizer_suppressions/ubsan

@@ -77,4 +77,6 @@ implicit-signed-integer-truncation:test/skiplist_tests.cpp
 implicit-signed-integer-truncation:torcontrol.cpp
 implicit-unsigned-integer-truncation:crypto/*
 implicit-unsigned-integer-truncation:leveldb/*
+# std::variant warning fixed in https://github.com/gcc-mirror/gcc/commit/074436cf8cdd2a9ce75cadd36deb8301f00e55b9
+implicit-unsigned-integer-truncation:std::__detail::__variant::_Variant_storage
 implicit-integer-sign-change:crc32c/*