Just wrote my own entity system to figure it out.

scratchpad/flecstest.cpp

@@ -0,0 +1,61 @@
+#include <flecs.h>
+#include <iostream>
+#include <typeindex>
+#include <typeinfo>
+#include <unordered_map>
+
+struct Position {
+    double x, y;
+};
+
+struct Velocity {
+    double x, y;
+};
+
+int main() {
+    flecs::world ecs;
+
+    flecs::system fight = ecs.system<const Position>()
+      .each([&](flecs::entity e, const Position &other) {
+          flecs::entity player = ecs.lookup("player");
+          if(e == player) return; // don't process the player
+
+          const Position *player_p = player.get<Position>();
+          if(player_p->x == other.x && player_p->y == other.y) {
+            std::cerr << "HIT\n";
+          }
+      });
+
+    // Create a system for Position, Velocity. Systems are like queries (see
+    // queries) with a function that can be ran or scheduled (see pipeline).
+    flecs::system move = ecs.system<Position, const Velocity>()
+        .each([&](flecs::entity e, Position& p, const Velocity& v) {
+            p.x += v.x;
+            p.y += v.y;
+            std::cerr << e.name() << ": {" << p.x << ", " << p.y << "}\n";
+        });
+
+    // Create a few test entities for a Position, Velocity query
+    ecs.entity("player")
+        .set<Position>({10, 10})
+        .set<Velocity>({1, 1});
+
+    ecs.entity("e2")
+      .set<Position>({10, 20})
+      .set<Velocity>({1, 1});
+
+    // This entity will not match as it does not have Position, Velocity
+    ecs.entity("e3")
+      .set<Position>({10, 20});
+
+    std::unordered_map<std::type_index, std::string> type_names;
+    type_names[std::type_index(typeid(Position))] = "Position";
+    type_names[std::type_index(typeid(Velocity))] = "Velocity";
+
+    for(const auto& [key, value] : type_names) {
+      std::cout << " VAL " << value << std::endl;
+    }
+    // Run the system
+    move.run();
+    fight.run();
+}