From fce3dd72783cd9991c0961905a155a689e8e073e Mon Sep 17 00:00:00 2001
From: Jacob Enders <jacobenders1@gmail.com>
Date: Sat, 8 Aug 2026 15:47:26 -0400
Subject: [PATCH] Block systems with tags in ADD_SYSTEM comptime function

---
 build.zig                    | 22 ++++++++++++++++++++++
 src/comptime_error_tests.zig | 31 +++++++++++++++++++++++++++++++
 src/zflecs.zig               |  7 +++++++
 3 files changed, 60 insertions(+)
 create mode 100644 src/comptime_error_tests.zig

diff --git a/build.zig b/build.zig
index ce74cf4..c4e4b70 100644
--- a/build.zig
+++ b/build.zig
@@ -683,4 +683,26 @@ pub fn build(b: *std.Build) void {
     b.installArtifact(tests);
 
     test_step.dependOn(&b.addRunArtifact(tests).step);
+
+    const comptime_test_step = b.step("test-comptime", "Run zflecs comptime tests");
+    const comptime_tests_module = b.createModule(.{
+        .root_source_file = b.path("src/comptime_error_tests.zig"),
+        .target = target,
+        .optimize = optimize,
+        .link_libc = true,
+    });
+    comptime_tests_module.addOptions("build-options", options);
+    comptime_tests_module.addIncludePath(b.path("libs/flecs"));
+    comptime_tests_module.linkLibrary(lib);
+
+    const comptime_tests = b.addTest(.{
+        .name = "zflecs-comptime-tests",
+        .root_module = comptime_tests_module,
+    });
+    comptime_tests.expect_errors = .{
+        .contains = "Tags are not allowed in system functions.",
+    };
+    b.installArtifact(comptime_tests);
+
+    comptime_test_step.dependOn(&comptime_tests.step);
 }
diff --git a/src/comptime_error_tests.zig b/src/comptime_error_tests.zig
new file mode 100644
index 0000000..cd818dd
--- /dev/null
+++ b/src/comptime_error_tests.zig
@@ -0,0 +1,31 @@
+const std = @import("std");
+const ecs = @import("zflecs.zig");
+const builtin = @import("builtin");
+
+const print = std.log.info;
+
+const Position = struct { x: f32, y: f32 };
+const Velocity = struct { x: f32, y: f32 };
+
+const Apples = struct {};
+
+fn move_apples_system(positions: []Position, velocities: []const Velocity, _: []const Apples) void {
+    for (positions, velocities) |*p, v| {
+        p.x += v.x;
+        p.y += v.y;
+    }
+}
+
+test "zflecs.block_tags_systemcomptime" {
+    print("\n", .{});
+
+    const world = ecs.init();
+    defer _ = ecs.fini(world);
+
+    ecs.COMPONENT(world, Position);
+    ecs.COMPONENT(world, Velocity);
+
+    ecs.TAG(world, Apples);
+
+    _ = ecs.ADD_SYSTEM(world, "move system", ecs.OnUpdate, move_apples_system);
+}
diff --git a/src/zflecs.zig b/src/zflecs.zig
index 2ece485..ef33aa5 100644
--- a/src/zflecs.zig
+++ b/src/zflecs.zig
@@ -3006,6 +3006,13 @@ pub fn SYSTEM_DESC(comptime fn_system: anytype) system_desc_t {
         const param_type_info = @typeInfo(p.type.?).pointer;
         const inout = if (param_type_info.is_const) .In else .InOut;
         system_desc.query.terms[i - start_index] = .{ .id = id(param_type_info.child), .inout = inout };
+
+        const child_info = @typeInfo(param_type_info.child).@"struct";
+        if (child_info.fields.len == 0) {
+            @compileError(
+                "Tags are not allowed in system functions.",
+            );
+        }
     }
 
     return system_desc;