Advanced TypeScript type-system mechanics — union and intersection types, conditional types with `infer`, mapped types, template literal types, generics and their constraints, index signatures, type assertions, and extracting types from other types (return types, array elements, promise results). Use when designing a complex generic, modeling a type that transforms based on another type, extracting a type from a function or structure with `infer`, or deciding between a type assertion and a real narrowing check.