How do you create a type that validates function argument relationships?

Use conditional types with infer to extract parameter types and validate relationships between arguments. Basic Parameter Validation: type ValidateArgs = T extends (...args: infer P) => any ? P : never; type FirstArg = T extends (first: infer F, ...rest: any[]) => any ? F : never; type SecondArg = T extends (first: any, second: infer S, ...rest: any[]) => any ? S : never; // Ensure second argument extends first type RelatedArgs = U extends T ? [T, U] : never; function processRelated(first: T, second: U): void { // Implementation } Advanced Validation: type ValidateFunctionArgs = F extends (...args: infer P) => any ? P extends [infer A, infer B] ? B extends A ? F : never : F : never; // Usage type ValidFunction = ValidateFunctionArgs<(base: string, extended: string) => void>; // OK type InvalidFunction = ValidateFunctionArgs<(base: string, extended: number) => void>; // never Benefits: • Compile-time argument validation • Ensures type relationships between parameters • Prevents invalid function signatures • Provides excellent type safety.