GCC plugin, detect 'typedef' and 'using'

Question

I'm writing a GCC plugin and would like to identify typedef and using.

plugin.cpp

#include <gcc-plugin.h>
#include <tree.h>
#include <tree-pretty-print.h>
#include <plugin-version.h>
#include <c-family/c-common.h>

#include <iostream>

int plugin_is_GPL_compatible;

static const std::string tree_code_str(enum tree_code code)
{
    std::string str {get_tree_code_name(code)};
    str += " (" + std::to_string(code) + ")";
    return str;
}

static const char* get_tree_name(tree t) {
    if (t == NULL_TREE) {
        return "<null>";
    }

    const char *name = get_name(t);
    if (name) {
        return name;
    }

    return "<no name>";
}

static const char* get_type_name_str(tree var_type)
{
    const char *type_name = NULL;
    if (TYPE_NAME(var_type) != NULL_TREE) {
        tree type_name_tree = TYPE_NAME(var_type);
        if (TREE_CODE(type_name_tree) == TYPE_DECL) {
            type_name = IDENTIFIER_POINTER(DECL_NAME(type_name_tree));
        } else if (TREE_CODE(type_name_tree) == IDENTIFIER_NODE) {
            type_name = IDENTIFIER_POINTER(type_name_tree);
        }
    }

    if (type_name) {
        return type_name;
    } else {
        return "<no type name>";
    }
}

static void plugin_finish_decl(void *gcc_data, void *user_data) {
    tree decl = (tree) gcc_data;
    std::cout << "TREE_CODE=[" << tree_code_str(TREE_CODE(decl)) << "] "
              << "TREE_TYPE=[" << get_type_name_str(TREE_TYPE(decl)) << "] "
              << "TREE_CODE(TREE_TYPE)=[" << tree_code_str(TREE_CODE(TREE_TYPE(decl))) << "] "
              << "TREE_CODE(TYPE_NAME)=[" << tree_code_str(TREE_CODE(TYPE_NAME(decl))) << "] "
              << "name=[" << get_tree_name(decl) << "] "
              << std::endl;
}

int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version) {
    if (!plugin_default_version_check(version, &gcc_version)) {
        fprintf(stderr, "Incompatible GCC version\n");
        return 1;
    }

    register_callback(plugin_info->base_name, PLUGIN_FINISH_DECL, plugin_finish_decl, NULL);

    return 0;
}

test.cpp

typedef int MyInt;
MyInt var {};
int main() { return 0; }

output

TREE_CODE=[var_decl (36)] TREE_TYPE=[MyInt] TREE_CODE(TREE_TYPE)=[integer_type (8)] TREE_CODE(TYPE_NAME)=[identifier_node (1)] name=[var]

As we can see from the output, MyInt is of type integer and not TYPE_DECL. GCC Documentation

I tried with GCC version 10 and 13. The result is the same.

Do you have an explanation? The GCC documentation doesn't seem clear on this point?