How is main() function not mangled in C++

When building a parser tree, GCC generates a "main" identifier node:

// in gcc/tree.h
#define main_identifier_node             global_trees[TI_MAIN_IDENTIFIER]
#define MAIN_NAME_P(NODE) \
  (IDENTIFIER_NODE_CHECK (NODE) == main_identifier_node)
// in gcc/c-family/c-common.c
main_identifier_node = get_identifier ("main");

GCC checks main and set it as a C declaration in gcc/cp/decl.c:

/* `main' and builtins have implicit 'C' linkage.  */
if (ctype == NULL_TREE
    && DECL_FILE_SCOPE_P (decl)
    && current_lang_name == lang_name_cplusplus
    && (MAIN_NAME_P (declarator)
    || (IDENTIFIER_LENGTH (declarator) > 10
        && IDENTIFIER_POINTER (declarator)[0] == '_'
        && IDENTIFIER_POINTER (declarator)[1] == '_'
        && strncmp (IDENTIFIER_POINTER (declarator)+2,
            "builtin_", 8) == 0)
    || (targetcm.cxx_implicit_extern_c
        && (targetcm.cxx_implicit_extern_c
        (IDENTIFIER_POINTER (declarator))))))
  SET_DECL_LANGUAGE (decl, lang_c);

Also, the compiler don't warn when main() doesn't have a return statement. This is also checked using MAIN_NAME_P macro.