Named and optional arguments are prevalent features in many mainstream programming languages, enhancing code readability and flexibility. Despite widespread use, their formalization has not been extensively studied. This paper bridges this gap by presenting a type-safe foundation for named and optional arguments using intersection and union types. We begin by identifying a critical type-safety issue in popular static type checkers for Python and Ruby, particularly in handling first-class named arguments in the presence of subtyping. Our solution involves rewriting call sites to ensure type safety, which we formalize through a translation into a core calculus called \(\lambda _\textsf{iu}\) . The type safety of the translation is proven using the Coq proof assistant. The practical implementation of our approach in the CP language validates our theoretical contributions. Furthermore, we informally discuss how our approach could be adapted to encode named and optional arguments in other existing languages.