depexts for test-only dependencies

[avsm]

I'm trying to add a depext that is only installed for a test installation in opam for ctypes:

depexts: [
  ["libffi-dev"] {os-distribution = "alpine"}
  ["fts-dev"] {os-distribution = "alpine" & with-test}

unfortunately, this doesn't work with opam list --external --with-test ctypes-foreign, and fts-dev is never installed. Is there some other way to get test-only depexts?

This is distinct from opam depext -t which installs the "mainline" depexts for dependent libraries required for a test suite. In this case, I only want to install libfts-dev to run the ctypes-foreign test suite, and not install it if the test suite isnt built.

[rjbou]

The option is only available as a package selector. Concerning the filter variable, it is not defined in the scope of depexts: field (cf. https://opam.ocaml.org/doc/Manual.html#Package-variables).
Maybe worth add it, plus with the incoming depext integration...
cc @AltGr

[avsm]

Thanks for the clarification; I'll add a conf-fts {with-test} package dependency to handle it for now, but it would be useful to have the filter variable in the upcoming integration indeed :-)

[AltGr]

In fact that would possibly add quite a lot of complexity, because with-* filters are resolved much later, at solver call time (they depend on the package and the command-line) ; and at the moment all depexts are computed (and used) at switch load time.