wgoyc's problems with the recently rewritten technology requirements code highlighted a couple of assumptions within the code of the structree, namely
- Assuming that a derived list of required techs doesn't contain a pair tech
- Assuming that just because a unit has a production component, that it actually possesses lists of units/technologies to train/research
This revision resolves these assumptions, and also adds an error to display in the case that a tech/unit/structure requires a technology that is not included in any of that civ's units' or structures' production lists.
Originally #4455
Also, see https://wildfiregames.com/forum/index.php?/topic/21573-ugh-okay-help/