Generate type traits for unions to map a type to the corresponding u… (#4032)

*  Generate type traits for unions to map a type to the corresponding union enum value.

* Fixed break with union enum type traits when type is in a namespace.

* Fixed spacing and variable names in type traits generation to match style guidelines.

*  Fixed spacing in type traits generation to match style guidelines.

* Regenerated test schema header.

src/idl_gen_cpp.cpp

@@ -483,7 +483,24 @@ class CppGenerator : public BaseGenerator {
       }
       code += "]; }\n\n";
     }
-
+    
+    // Generate type traits for unions to map from a type to union enum value.
+    if (enum_def.is_union) {
+      for (auto it = enum_def.vals.vec.begin();
+        it != enum_def.vals.vec.end();
+        ++it) {
+        auto &ev = **it;
+        if (it == enum_def.vals.vec.begin()) {
+          code += "template<typename T> struct " + enum_def.name + "Traits {\n";
+        }
+        else {
+          code += "template<> struct " + enum_def.name + "Traits<" + WrapInNameSpace(*ev.struct_def) + "> {\n";
+        }
+        code += "  static const " + enum_def.name + " enum_value = " + GenEnumValDecl(enum_def, ev.name, parser_.opts) + ";\n";
+        code += "};\n\n";
+      }
+    }
+    
     if (enum_def.is_union) {
       code += UnionVerifySignature(enum_def) + ";\n\n";
     }

tests/monster_test_generated.h

@@ -76,6 +76,22 @@ inline const char **EnumNamesAny() {
 
 inline const char *EnumNameAny(Any e) { return EnumNamesAny()[static_cast<int>(e)]; }
 
+template<typename T> struct AnyTraits {
+  static const Any enum_value = Any_NONE;
+};
+
+template<> struct AnyTraits<Monster> {
+  static const Any enum_value = Any_Monster;
+};
+
+template<> struct AnyTraits<TestSimpleTableWithEnum> {
+  static const Any enum_value = Any_TestSimpleTableWithEnum;
+};
+
+template<> struct AnyTraits<MyGame::Example2::Monster> {
+  static const Any enum_value = Any_MyGame_Example2_Monster;
+};
+
 inline bool VerifyAny(flatbuffers::Verifier &verifier, const void *union_obj, Any type);
 
 MANUALLY_ALIGNED_STRUCT(2) Test FLATBUFFERS_FINAL_CLASS {