Adding readonly inside the mapped type rebuild marks every property on the new shape as immutable. The conditional T extends object ? ... : T stops the recursion at primitives, just like the DeepPartial pattern. The cast in the runtime sentinel is for testing only; in real code the value comes from a function that returns DeepReadonlyBasic<T> so the compiler tracks immutability transitively. Note that readonly is a compile-time check only; it does not freeze the object at runtime, which the third accordion fixes.

The basic shape converts arrays to a record-of-readonly-properties, which loses the [] index signature plus all the array methods. Branching on Array<infer U> first preserves the array shape and converts it to ReadonlyArray<...>, which removes mutating methods (push, pop, splice) from the type while keeping map, filter, etc. The infer U in the conditional matches the element type. Tuples follow the same path because Array<infer U> matches them, though for a strict tuple-preserving variant you can pattern-match T extends [infer A, ...infer R].

TypeScript's readonly is erased at runtime, so a determined caller can still mutate the object via a cast. Walking the value tree and calling Object.freeze on every node enforces the contract at runtime: in strict mode the assignment throws, in sloppy mode it silently no-ops. The signature <T>(value: T): DeepReadonly<T> is the headline trick because callers get the static guarantee and the dynamic guarantee from one call. Do not freeze hot paths in performance-sensitive code; freezing has measurable overhead and is best applied at module boundaries (configuration, store snapshots, public exports).