fix [firststrike] special with apply_to=opponent crashes Wesnoth 1.17.x

data/test/scenarios/poison_opponent.cfg

@@ -0,0 +1,114 @@
+# this unit test what 'poison' applied to opponent work perfectly after fixing.
+{GENERIC_UNIT_TEST "poison_opponent" (
+    [event]
+        name=start
+        [modify_unit]
+            [filter]
+            [/filter]
+            max_hitpoints=100
+            hitpoints=100
+            attacks_left=1
+        [/modify_unit]
+        [object]
+            silent=yes
+            [effect]
+                apply_to=attack
+                [set_specials]
+                    mode=append
+                    [poison]
+                        apply_to=opponent
+                    [/poison]
+                    [attacks]
+                        value=1
+                    [/attacks]
+                    [damage]
+                        value=1
+                    [/damage]
+                    [chance_to_hit]
+                        value=100
+                    [/chance_to_hit]
+                [/set_specials]
+            [/effect]
+            [filter]
+                id=bob
+            [/filter]
+        [/object]
+        [object]
+            silent=yes
+            [effect]
+                apply_to=attack
+                [set_specials]
+                    mode=append
+                    [poison]
+                        apply_to=opponent
+                    [/poison]
+                    [attacks]
+                        value=1
+                    [/attacks]
+                    [damage]
+                        value=1
+                    [/damage]
+                    [chance_to_hit]
+                        value=100
+                    [/chance_to_hit]
+                [/set_specials]
+            [/effect]
+            [filter]
+                id=alice
+            [/filter]
+        [/object]
+
+        [store_unit]
+            [filter]
+                id=alice
+            [/filter]
+            variable=a
+            kill=yes
+        [/store_unit]
+        [store_unit]
+            [filter]
+                id=bob
+            [/filter]
+            variable=b
+        [/store_unit]
+        [unstore_unit]
+            variable=a
+            find_vacant=yes
+            x,y=$b.x,$b.y
+        [/unstore_unit]
+        [store_unit]
+            [filter]
+                id=alice
+            [/filter]
+            variable=a
+        [/store_unit]
+
+        [do_command]
+            [attack]
+                weapon=0
+                defender_weapon=0
+                [source]
+                    x,y=$a.x,$a.y
+                [/source]
+                [destination]
+                    x,y=$b.x,$b.y
+                [/destination]
+            [/attack]
+        [/do_command]
+        [store_unit]
+            [filter]
+                id=alice
+            [/filter]
+            variable=a
+        [/store_unit]
+        [store_unit]
+            [filter]
+                id=bob
+            [/filter]
+            variable=b
+        [/store_unit]
+        {ASSERT ({VARIABLE_CONDITIONAL a.status.poisoned boolean_equals yes})}
+        {ASSERT ({VARIABLE_CONDITIONAL b.status.poisoned boolean_equals yes})}
+        {SUCCEED}
+    [/event]
+)}

src/units/abilities.cpp

@@ -1537,29 +1537,30 @@ bool attack_type::special_active_impl(const_attack_ptr self_attack, const_attack
 	temporary_facing self_facing(self, self_loc.get_relative_dir(other_loc));
 	temporary_facing other_facing(other, other_loc.get_relative_dir(self_loc));
 
-	// Filter poison, plague, drain, first strike
-	if (tag_name == "drains" && other && other->get_state("undrainable")) {
-		return false;
-	}
-	if (tag_name == "plague" && other &&
-		(other->get_state("unplagueable") ||
-		 resources::gameboard->map().is_village(other_loc))) {
+	// Filter poison, plague, drain, slow, petrifies
+	// True if "whom" corresponds to "self", false if "whom" is "other"
+	bool whom_is_self = ((whom == AFFECT_SELF) || ((whom == AFFECT_EITHER) && special_affects_self(special, is_attacker))) ? true : false;
+	unit_const_ptr whom_is_unit = whom_is_self ? other : self;
+	map_location whom_is_loc = whom_is_self ? other_loc : self_loc;
+
+	if (tag_name == "drains" && whom_is_unit && whom_is_unit->get_state("undrainable")) {
 		return false;
 	}
-	if (tag_name == "poison" && other &&
-		(other->get_state("unpoisonable") || other->get_state(unit::STATE_POISONED))) {
+	if (tag_name == "plague" && whom_is_unit &&
+		(whom_is_unit->get_state("unplagueable") ||
+		 resources::gameboard->map().is_village(whom_is_loc))) {
 		return false;
 	}
-	if (tag_name == "firststrike" && !is_attacker && other_attack &&
-		other_attack->has_special_or_ability("firststrike")) {
+	if (tag_name == "poison" && whom_is_unit &&
+		(whom_is_unit->get_state("unpoisonable") || whom_is_unit->get_state(unit::STATE_POISONED))) {
 		return false;
 	}
-	if (tag_name == "slow" && other &&
-		(other->get_state("unslowable") || other->get_state(unit::STATE_SLOWED))) {
+	if (tag_name == "slow" && whom_is_unit &&
+		(whom_is_unit->get_state("unslowable") || whom_is_unit->get_state(unit::STATE_SLOWED))) {
 		return false;
 	}
-	if (tag_name == "petrifies" && other &&
-		other->get_state("unpetrifiable")) {
+	if (tag_name == "petrifies" && whom_is_unit &&
+		whom_is_unit->get_state("unpetrifiable")) {
 		return false;
 	}
 
@@ -1572,6 +1573,14 @@ bool attack_type::special_active_impl(const_attack_ptr self_attack, const_attack
 	const_attack_ptr att_weapon = is_attacker ? self_attack : other_attack;
 	const_attack_ptr def_weapon = is_attacker ? other_attack : self_attack;
 
+	// Filter firststrike here, if both units have first strike then the effects cancel out. Only check
+	// the opponent if "whom" is the defender, otherwise this leads to infinite recursion.
+	if (tag_name == "firststrike") {
+		bool whom_is_defender = whom_is_self ? !is_attacker : is_attacker;
+		if (whom_is_defender && att_weapon && att_weapon->has_special_or_ability("firststrike"))
+			return false;
+	}
+
 	// Filter the units involved.
 	if (!special_unit_matches(self, other, self_loc, self_attack, special, is_for_listing, filter_self))
 		return false;

wml_test_schedule

@@ -267,6 +267,7 @@
 0 swarm_disables_upgrades_with_abilities_adjacent_fail
 0 swarm_disables_upgrades_with_abilities_adjacent_leadership
 0 swarm_disables_upgrades_with_abilities_adjacent_leadership_fail
+0 poison_opponent
 0 test_add_in_leadership_abilities
 0 test_sub_in_leadership_abilities
 0 unslowable_status_test