Module#define_method

static VALUE
rb_mod_define_method(int argc, VALUE *argv, VALUE mod)
{
    ID id;
    VALUE body;
    VALUE name;
    const rb_cref_t *cref = rb_vm_cref_in_context(mod, mod);
    const rb_scope_visibility_t default_scope_visi = {METHOD_VISI_PUBLIC, FALSE};
    const rb_scope_visibility_t *scope_visi = &default_scope_visi;
    int is_method = FALSE;

    if (cref) {
        scope_visi = CREF_SCOPE_VISI(cref);
    }

    rb_check_arity(argc, 1, 2);
    name = argv[0];
    id = rb_check_id(&name);
    if (argc == 1) {
        body = rb_block_lambda();
    }
    else {
        body = argv[1];

        if (rb_obj_is_method(body)) {
            is_method = TRUE;
        }
        else if (rb_obj_is_proc(body)) {
            is_method = FALSE;
        }
        else {
            rb_raise(rb_eTypeError,
                     "wrong argument type %s (expected Proc/Method/UnboundMethod)",
                     rb_obj_classname(body));
        }
    }
    if (!id) id = rb_to_id(name);

    if (is_method) {
        struct METHOD *method = (struct METHOD *)DATA_PTR(body);
        if (method->me->owner != mod && !RB_TYPE_P(method->me->owner, T_MODULE) &&
            !RTEST(rb_class_inherited_p(mod, method->me->owner))) {
            if (FL_TEST(method->me->owner, FL_SINGLETON)) {
                rb_raise(rb_eTypeError,
                         "can't bind singleton method to a different class");
            }
            else {
                rb_raise(rb_eTypeError,
                         "bind argument must be a subclass of % "PRIsVALUE,
                         method->me->owner);
            }
        }
        rb_method_entry_set(mod, id, method->me, scope_visi->method_visi);
        if (scope_visi->module_func) {
            rb_method_entry_set(rb_singleton_class(mod), id, method->me, METHOD_VISI_PUBLIC);
        }
        RB_GC_GUARD(body);
    }
    else {
        VALUE procval = rb_proc_dup(body);
        if (vm_proc_iseq(procval) != NULL) {
            rb_proc_t *proc;
            GetProcPtr(procval, proc);
            proc->is_lambda = TRUE;
            proc->is_from_method = TRUE;
        }
        rb_add_method(mod, id, VM_METHOD_TYPE_BMETHOD, (void *)procval, scope_visi->method_visi);
        if (scope_visi->module_func) {
            rb_add_method(rb_singleton_class(mod), id, VM_METHOD_TYPE_BMETHOD, (void *)body, METHOD_VISI_PUBLIC);
        }
    }

    return ID2SYM(id);
}