Testing - Can we mock a service with package level exported variable?

From this below line of code, package level exported variable from unit testing code is overwriting package level exported variable of service layer, basically to change the service(to use).

  services.MessagesService = &serviceMock{} // overwrite with another mock service

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Given the scenario, that we do not introduce more than one implementation of messageServiceInterface by hard coding a service(MessagesService messageServiceInterface = &messagesService{}) as mentioned here:

Does overwriting package level exported variable approach cause side affects in regular functional flow?