Allow type info to be used as a key. (#3822)

src/ngraph/type.hpp

@@ -17,8 +17,11 @@
 #pragma once
 
 #include <memory>
+#include <string>
 #include <utility>
 
+#include "ngraph/ngraph_visibility.hpp"
+
 namespace ngraph
 {
     /// Supports three functions, is_type<Type>, as_type<Type>, and as_type_ptr<Type> for type-safe
@@ -28,12 +31,19 @@ namespace ngraph
 
     /// Type information for a type system without inheritance; instances have exactly one type not
     /// related to any other type.
+    NGRAPH_API
     struct DiscreteTypeInfo
     {
         const char* name;
         uint64_t version;
 
         bool is_castable(const DiscreteTypeInfo& target_type) const { return this == &target_type; }
+        // For use as a key
+        bool operator<(const DiscreteTypeInfo& b) const
+        {
+            return version < b.version ||
+                   (version == b.version && std::string(name) < std::string(b.name));
+        }
     };
 
     /// \brief Tests if value is a pointer/shared_ptr that can be statically cast to a