error in test

I use your modbus lib in my project, but when I test it:

> dub test -q                                                                                          master ⬆ ◼
../../../.dub/packages/serialport-2.2.3/serialport/source/serialport/util.d(59,6): Error: typesafe variadic function parameter `list` of type `Pair!(int, int)[]` cannot be marked `return`
/usr/bin/dmd failed with exit code 1.

> dmd --version                                                                                    ✘ 2 master ⬆ ◼
DMD64 D Compiler v2.100.0

but if I test your source:

> cd ~/d/serialport/                                                                                   master ⬆ ◼
> git describe                                                                                             master
v2.2.3
> dub test -q                                                                                              master
=== start real test ===
...
> cd ~/d/serialport/                                                                                   master ⬆ ◼
[I] [10:13:50] o3o@forty /home/o3o/d/serialport
> git describe                                                                                             master
v2.2.3
[I] [10:13:57] o3o@forty /home/o3o/d/serialport
> dub test -q                                                                                              master
=== start real test ===
...
<<< success fiber sleep func test
=== finish real test ===
4 modules passed unittests

odd...

So, my project use dip1000 and as Steven wrote

One of the main drawbacks of dip1000 is that if you don't have it enabled on ALL projects, you will likely run into linker errors. It's like compiling 2 libraries with different versions and expecting them to link. It's probably the reason dub forwards the switches.