(Sub)module not found in test when using dune

Hi, I’m new to OCaml and I’m trying to understand how to use dune.

I constructed a minimal example that looks like this:

.
├── dune-project
├── lib
│   ├── dune
│   ├── mylib.ml
│   └── mymod.ml
└── test
    ├── dune
    └── test.ml

The dune-project file just contains one line: (lang dune 3.12)

The lib/dune file contains: (library (name mylib))

The test/dune file contains:

(test
 (name test)
 (libraries mylib))

The lib/mylib.ml file contains:

let one = 1
let _ = assert (one = 1)
let _ = assert (Mymod.two = 2)

And the lib/mymod.ml file, which is supposed to be a submodule, contains:

let two = 2

Now my test in test/test.ml looks like this:

let _ = assert (Mylib.one = 1)
let _ = assert (Mylib.Mymod.two = 2)

I get the following error:

$ dune test
File "test/test.ml", line 2, characters 16-31:
2 | let _ = assert (Mylib.Mymod.two = 2)
                    ^^^^^^^^^^^^^^^
Error: Unbound module Mylib.Mymod

Why does Mymod.two work inside lib/mylib.ml but not inside the test. What am I doing wrong?

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