Create comparator wrapper classes for MSVC 2019 build issues:

* The std::less and std::equal_to comparators have [[nodiscard]], which
  conflicts with boost::bimap validations.

src/ripple/basics/comparators.h

@@ -0,0 +1,76 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012, 2013 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef RIPPLE_BASICS_COMPARATORS_H_INCLUDED
+#define RIPPLE_BASICS_COMPARATORS_H_INCLUDED
+
+#include <functional>
+
+namespace ripple {
+
+#ifdef _MSC_VER
+
+/*
+ * MSVC 2019 version 16.9.0 added [[nodiscard]] to the std comparison
+ * operator() functions. boost::bimap checks that the comparitor is a
+ * BinaryFunction, in part by calling the function and ignoring the value.
+ * These two things don't play well together. These wrapper classes simply
+ * strip [[nodiscard]] from operator() for use in boost::bimap.
+ *
+ * See also:
+ *  https://www.boost.org/doc/libs/1_75_0/libs/bimap/doc/html/boost_bimap/the_tutorial/controlling_collection_types.html
+ */
+
+template <class T = void>
+struct less
+{
+    using result_type = bool;
+
+    constexpr bool
+    operator()(const T& left, const T& right) const
+    {
+        return std::less<T>()(left, right);
+    }
+};
+
+template <class T = void>
+struct equal_to
+{
+    using result_type = bool;
+
+    constexpr bool
+    operator()(const T& left, const T& right) const
+    {
+        return std::equal_to<T>()(left, right);
+    }
+};
+
+#else
+
+template <class T = void>
+using less = std::less<T>;
+
+template <class T = void>
+using equal_to = std::equal_to<T>;
+
+#endif
+
+}  // namespace ripple
+
+#endif

src/ripple/peerfinder/impl/Bootcache.h

@@ -20,6 +20,7 @@
 #ifndef RIPPLE_PEERFINDER_BOOTCACHE_H_INCLUDED
 #define RIPPLE_PEERFINDER_BOOTCACHE_H_INCLUDED
 
+#include <ripple/basics/comparators.h>
 #include <ripple/beast/utility/Journal.h>
 #include <ripple/beast/utility/PropertyStream.h>
 #include <ripple/peerfinder/PeerfinderManager.h>
@@ -81,8 +82,11 @@ class Bootcache
         int m_valence;
     };
 
-    using left_t = boost::bimaps::unordered_set_of<beast::IP::Endpoint>;
-    using right_t = boost::bimaps::multiset_of<Entry>;
+    using left_t = boost::bimaps::unordered_set_of<
+        beast::IP::Endpoint,
+        boost::hash<beast::IP::Endpoint>,
+        ripple::equal_to<beast::IP::Endpoint>>;
+    using right_t = boost::bimaps::multiset_of<Entry, ripple::less<Entry>>;
     using map_type = boost::bimap<left_t, right_t>;
     using value_type = map_type::value_type;
 

src/test/csf/ledgers.h

@@ -21,6 +21,7 @@
 
 #include <ripple/basics/UnorderedContainers.h>
 #include <ripple/basics/chrono.h>
+#include <ripple/basics/comparators.h>
 #include <ripple/basics/tagged_integer.h>
 #include <ripple/consensus/LedgerTiming.h>
 #include <ripple/json/json_value.h>
@@ -242,7 +243,9 @@ class Ledger
  */
 class LedgerOracle
 {
-    using InstanceMap = boost::bimaps::bimap<Ledger::Instance, Ledger::ID>;
+    using InstanceMap = boost::bimaps::bimap<
+        boost::bimaps::set_of<Ledger::Instance, ripple::less<Ledger::Instance>>,
+        boost::bimaps::set_of<Ledger::ID, ripple::less<Ledger::ID>>>;
     using InstanceEntry = InstanceMap::value_type;
 
     // Set of all known ledgers; note this is never pruned