Freeciv

Currently, there are very few checks for if requirements are valid for a specific effect. In fact, many requirements can be redundant since the effect is calculated without checking the object they refer to (e.g., as Wahazar noticed here, a city can't directly contribute into National_History, because the function is get_player_bonus(pplayer, EFT_NATION_HISTORY)) or checks not the thing one would think it does (e.g. no effects other than Unit_Bribe_Cost_Pct are interested in a unit's home city). Since effect reqs are calculated to RPT_CERTAIN on the server, that means one silently does not get what was desired. I did document effect factors on Fandom but I think it would be much better if sanity checks catched what is possible on ruleset load.

Likely, we make

#define SPECENUM_NAME req_factor
#define SPECENUM_VALUE0 RQP_PLAYER
#define SPECENUM_VALUE0NAME "player" 
#define SPECENUM_VALUE1 RQP_OTHER_PLAYER
#define SPECENUM_VALUE1NAME "other player" 
#define SPECENUM_VALUE2 RQP_CITY
#define SPECENUM_VALUE2NAME "city" 
#define SPECENUM_VALUE3 RQP_IMPR_TYPE
#define SPECENUM_VALUE3NAME "improvement" 
#define SPECENUM_VALUE4 RQP_TILE
#define SPECENUM_VALUE4NAME "tile" 
#define SPECENUM_VALUE5 RQP_UNIT
#define SPECENUM_VALUE5NAME "unit" 
#define SPECENUM_VALUE6 RQP_UNIT_TYPE
#define SPECENUM_VALUE6NAME "unit type" 
#define SPECENUM_VALUE7 RQP_OUTPUT_TYPE
#define SPECENUM_VALUE7NAME "output type" 
#define SPECENUM_VALUE8 RQP_SPECIALIST
#define SPECENUM_VALUE8NAME "specialist" 
#define SPECENUM_VALUE9 RQP_ACTION
#define SPECENUM_VALUE9NAME "action" 
#define SPECENUM_COUNT RQP_COUNT
#define SPECENUM_BITVECTOR bv_req_factors
#include "specenum_gen.h" 

static bv_req_factors req_factors[REQ_RANGE_COUNT][VUT_COUNT] = ...

static bv_req_factors effect_factors[EFT_COUNT] = ...

And check that each req_factors[req][range] is subset of effect_factors[eft]. This requires much work but allows also to simplify req_from_str().