merge typing is wrong

[radarsu]

Typing of merge function:

declare function merge<T extends Record<PropertyKey, any>, S extends Record<PropertyKey, any>>(target: T, source: S): T & S;

Does not describe a true deep merge. It only says: “return something that has all properties of T and S.” This is a shallow structural type and does not capture recursive merging behavior.

I suggest using following instead:

export type OverrideDeep<Base, Override> = Base extends object
    ? Override extends object
        ? {
              [K in keyof Base | keyof Override]: K extends keyof Override
                  ? K extends keyof Base
                      ? OverrideDeep<Base[K], Override[K]>
                      : Override[K]
                  : K extends keyof Base
                    ? Base[K]
                    : never;
          }
        : Override
    : Override;