registry2.zig

const Node = struct {
    t: type = bool,
    next: ?*Node = null,
};

var registry: ?*Node = null;

fn addComponent(comptime T: type) void {
    registry = Node{ .t = T, .next = registry };
}

test "add component" {
    const Mover = struct {
        foo: bool,
    };

    comptime addComponent(Mover);
}

// RESULT:
//
//registry.zig:9:38: error: cannot store runtime value in type '?*Node'
//    registry = Node{ .t = T, .next = registry };
//                                     ^
//registry.zig:17:26: note: called from here
//    comptime addComponent(Mover);
                         ^