dune-variant issue

Reproduction requires a few packages:

• ocaml (4.14.2 - it's not working with 5 right now)
• dune (3.15.2 - should not matter)
• ocaml-solo5 (0.8.4 - but should not matter)

So, opam sw create 4.14.2 dune-variant-repro ; eval opam env ; opam install ocaml-solo5 dune

I suspect the ocaml-solo5 is needed for the findlib.conf.

Run the build

This leads to the original issue reported in ocaml/dune#10460

$ dune bu dist
File "dune.build", line 13, characters 40-60:
13 |  (libraries duration lwt mirage-bootvar mirage-bootvar.solo5
                                             ^^^^^^^^^^^^^^^^^^^^
Error: Library "mirage-bootvar.solo5" in
_build/solo5/duniverse/mirage-bootvar/solo5 is hidden (optional with
unavailable dependencies).
-> required by _build/solo5/main.exe
-> required by alias all (context solo5)
-> required by alias default (context solo5)

Change proposed by Rudi in ocaml/dune#10460 (comment) doesn't result in different behaviour.

Remove "(optional)" from duniverse/mirage-bootvar/solo5/dune

The build results in a different error:

File "duniverse/mirage-time/unix/dune", line 5, characters 12-20:
5 |  (libraries lwt.unix duration))
                ^^^^^^^^
Error: Library "lwt.unix" not found.
-> required by library "mirage-time.unix" in
   _build/solo5/duniverse/mirage-time/unix
-> required by library "mirage-bootvar.solo5" in
   _build/solo5/duniverse/mirage-bootvar/solo5
-> required by executable main in dune.build:11
-> required by _build/solo5/main.exe
-> required by _build/solo5/hello.hvt
-> required by _build/solo5/dist/hello.hvt
-> required by alias dist/all (context solo5)
-> required by alias dist/default (context solo5)

Remove default_implementation from duniverse/mirage-time/src/dune

There's a different error:

Error: No implementation found for virtual library "mirage-time" in
_build/solo5/duniverse/mirage-time/src.
-> required by library "mirage-solo5" in
   _build/solo5/duniverse/mirage-solo5/lib
-> required by library "mirage-bootvar.solo5" in
   _build/solo5/duniverse/mirage-bootvar/solo5
-> required by executable main in dune.build:11
-> required by _build/solo5/main.exe
-> required by _build/solo5/hello.hvt
-> required by _build/solo5/dist/hello.hvt
-> required by alias dist/all (context solo5)
-> required by alias dist/default (context solo5)

Dirty fix

Change in duniverse/mirage-time/src/dune the (default_implementation mirage-time.unix) to (default_implementation mirage-time.solo5).

Dependencies

In dune.build, the dependencies include: mirage-bootvar.solo5, mirage-time.solo5

Both are implementations of the virtual_modules (mirage-bootvar, mirage-time).

The package mirage-solo5 depends on mirage-time; the package mirage-bootvar depends on mirage-solo5.

It looks like during building mirage-bootvar.solo5 the specification of the implementation of mirage-time is not taken into consideration.