It seems like you have a typo. The class name should be ProductTemplate if you are trying to override product.template.

我在ODOO【18.0】中也有一样的需求，根据此贴将default_coder compute设置成False后解决，十分感谢。

Hey all to override a computed method we need to define the field again in the inherited class

class ProductTemplate(models.Model):
    _inherit = 'product.template'
    default_code = field.Char(compute='_compute_default_code')
    @api.one                                                                                                                                                                                                                                                                                                                                                                                                                                          
    @api.depends('product_variant_ids', 'product_variant_ids.default_code')
    def _compute_default_code(self):
        print("overridden")