Add util::Ref class as temporary alternative for c++17 std::any

This commit does not change behavior

src/util/check.h

@@ -5,6 +5,10 @@
 #ifndef BITCOIN_UTIL_CHECK_H
 #define BITCOIN_UTIL_CHECK_H
 
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
 #include <tinyformat.h>
 
 #include <stdexcept>

src/util/ref.h

@@ -0,0 +1,38 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_REF_H
+#define BITCOIN_UTIL_REF_H
+
+#include <util/check.h>
+
+#include <typeindex>
+
+namespace util {
+
+/**
+ * Type-safe dynamic reference.
+ *
+ * This implements a small subset of the functionality in C++17's std::any
+ * class, and can be dropped when the project updates to C++17
+ * (https://github.com/bitcoin/bitcoin/issues/16684)
+ */
+class Ref
+{
+public:
+    Ref() = default;
+    template<typename T> Ref(T& value) { Set(value); }
+    template<typename T> T& Get() const { CHECK_NONFATAL(Has<T>()); return *static_cast<T*>(m_value); }
+    template<typename T> void Set(T& value) { m_value = &value; m_type = std::type_index(typeid(T)); }
+    template<typename T> bool Has() const { return m_value && m_type == std::type_index(typeid(T)); }
+    void Clear() { m_value = nullptr; m_type = std::type_index(typeid(void)); }
+
+private:
+    void* m_value = nullptr;
+    std::type_index m_type = std::type_index(typeid(void));
+};
+
+} // namespace util
+
+#endif // BITCOIN_UTIL_REF_H