feat(C++): Support underlying_type for union (#7954)

* feat(C++): support underlying type for union

* chore: add conform checks for underlying type changes

src/idl_gen_cpp.cpp

@@ -779,7 +779,12 @@ class CppGenerator : public BaseGenerator {
       if (type.enum_def) return WrapInNameSpace(*type.enum_def);
       if (type.base_type == BASE_TYPE_BOOL) return "bool";
     }
-    return StringOf(type.base_type);
+    // Get real underlying type for union type 
+    auto base_type = type.base_type;
+    if (type.base_type == BASE_TYPE_UTYPE && type.enum_def != nullptr) {
+        base_type = type.enum_def->underlying_type.base_type;
+    }
+    return StringOf(base_type);
   }
 
   // Return a C++ pointer type, specialized to the actual struct/table types,
@@ -1048,7 +1053,7 @@ class CppGenerator : public BaseGenerator {
 
   std::string UnionVectorVerifySignature(const EnumDef &enum_def) {
     const std::string name = Name(enum_def);
-    const std::string &type = opts_.scoped_enums ? name : "uint8_t";
+    const std::string &type = opts_.scoped_enums ? name : GenTypeBasic(enum_def.underlying_type, false); 
     return "bool Verify" + name + "Vector" +
            "(::flatbuffers::Verifier &verifier, " +
            "const ::flatbuffers::Vector<::flatbuffers::Offset<void>> "
@@ -3496,12 +3501,13 @@ class CppGenerator : public BaseGenerator {
           }
           case BASE_TYPE_UTYPE: {
             value = StripUnionType(value);
+            auto underlying_type = GenTypeBasic(vector_type, false);
             const std::string &type = opts_.scoped_enums
                                           ? Name(*field.value.type.enum_def)
-                                          : "uint8_t";
+                                          : underlying_type;
             auto enum_value = "__va->_" + value + "[i].type";
             if (!opts_.scoped_enums)
-              enum_value = "static_cast<uint8_t>(" + enum_value + ")";
+              enum_value = "static_cast<" + underlying_type + ">(" + enum_value + ")";
 
             code += "_fbb.CreateVector<" + type + ">(" + value +
                     ".size(), [](size_t i, _VectorArgs *__va) { return " +

src/idl_parser.cpp

@@ -947,8 +947,12 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
   if (type.base_type == BASE_TYPE_UNION) {
     // For union fields, add a second auto-generated field to hold the type,
     // with a special suffix.
-    ECHECK(AddField(struct_def, name + UnionTypeFieldSuffix(),
-                    type.enum_def->underlying_type, &typefield));
+
+    // To ensure compatibility with many codes that rely on the BASE_TYPE_UTYPE value to identify union type fields.
+    Type union_type(type.enum_def->underlying_type);
+    union_type.base_type = BASE_TYPE_UTYPE;
+    ECHECK(AddField(struct_def, name + UnionTypeFieldSuffix(),union_type, &typefield));
+    
   } else if (IsVector(type) && type.element == BASE_TYPE_UNION) {
     advanced_features_ |= reflection::AdvancedUnionFeatures;
     // Only cpp, js and ts supports the union vector feature so far.
@@ -2482,23 +2486,39 @@ CheckedError Parser::ParseEnum(const bool is_union, EnumDef **dest,
         &GetPooledString(RelativeToRootPath(opts.project_root, filename));
   }
   enum_def->doc_comment = enum_comment;
-  if (!is_union && !opts.proto_mode) {
+  if (!opts.proto_mode) {
     // Give specialized error message, since this type spec used to
     // be optional in the first FlatBuffers release.
+    bool explicit_underlying_type = false;
     if (!Is(':')) {
-      return Error(
-          "must specify the underlying integer type for this"
-          " enum (e.g. \': short\', which was the default).");
+      // Enum is forced to have an explicit underlying type in declaration.
+      if (!is_union) {
+        return Error(
+            "must specify the underlying integer type for this"
+            " enum (e.g. \': short\', which was the default).");
+      }
     } else {
+      // Union underlying type is only supported for cpp
+      if (is_union && !SupportsUnionUnderlyingType()) {
+        return Error(
+            "Underlying type for union is not yet supported in at least one of "
+            "the specified programming languages.");
+      }
       NEXT();
+      explicit_underlying_type = true;
     }
-    // Specify the integer type underlying this enum.
-    ECHECK(ParseType(enum_def->underlying_type));
-    if (!IsInteger(enum_def->underlying_type.base_type) ||
-        IsBool(enum_def->underlying_type.base_type))
-      return Error("underlying enum type must be integral");
-    // Make this type refer back to the enum it was derived from.
-    enum_def->underlying_type.enum_def = enum_def;
+
+    if (explicit_underlying_type) {
+      // Specify the integer type underlying this enum.
+      ECHECK(ParseType(enum_def->underlying_type));
+      if (!IsInteger(enum_def->underlying_type.base_type) || IsBool(enum_def->underlying_type.base_type)) {
+        return Error("underlying " + std::string(is_union ? "union" : "enum") + "type must be integral");
+      }
+        
+      // Make this type refer back to the enum it was derived from.
+      enum_def->underlying_type.enum_def = enum_def;
+    }
+
   }
   ECHECK(ParseMetaData(&enum_def->attributes));
   const auto underlying_type = enum_def->underlying_type.base_type;
@@ -2697,6 +2717,10 @@ bool Parser::Supports64BitOffsets() const {
           ~(IDLOptions::kCpp | IDLOptions::kJson | IDLOptions::kBinary)) == 0;
 }
 
+bool Parser::SupportsUnionUnderlyingType() const {
+    return (opts.lang_to_generate & ~IDLOptions::kCpp) == 0;
+}
+
 Namespace *Parser::UniqueNamespace(Namespace *ns) {
   for (auto it = namespaces_.begin(); it != namespaces_.end(); ++it) {
     if (ns->components == (*it)->components) {
@@ -4428,6 +4452,10 @@ std::string Parser::ConformTo(const Parser &base) {
           return "values differ for enum: " + enum_val.name;
       }
     }
+    // Check underlying type changes
+    if (enum_def_base->underlying_type.base_type != enum_def.underlying_type.base_type) {
+      return "underlying type differ for " + std::string(enum_def.is_union ? "union: " : "enum: ") + qualified_name;
+    }
   }
   return "";
 }