Incorrect removal of a package with conditional dependency

[rjbou]

If you have a switch with dune & ocaml < 4.08, you automatically have ocamlfind-secondary installed, but if you upgrade your ocaml, you can't remove ocamlfind-secondary without removing dune.
dune.2.8.5 have:

depends      ("ocaml" {>= "4.08"} | ("ocaml" {< "4.08~~"} & "ocamlfind-secondary"))

Calculation of reverse dependencies for automatic removal is too wide, it includes packages from rev deps disjunctions.

[AltGr]

The incriminated reverse dependency calculation is here:

opam/src/client/opamClient.ml

Lines 1374 to 1377 in 82fb1ad

	let to_remove =
	OpamSolver.reverse_dependencies ~build:true ~post:true
	~depopts:false ~installed:true universe packages
	in

This is useful when using the "autoremove" flag.

We could try to have a separate computation for "guaranteed" reverse dependencies (rather than the "inclusive" set now computed) but that would be quite involved and trigger other weirdnesses. The proper solution would probably be, instead, to remove this preprocessing and let the solver do the work: define an internal Cudf package flag "manually-installed" and adjust the solver criteria to something in the lines of -count[changed,manually-installed],+removed,...

[rjbou]

@rjbou add a test

[AltGr]

From a quick test:

• on master, this is solved in the case of a simple opam remove:

  % opam remove --show ocamlfind-secondary
  The following actions would be faked:
    ⊘ remove    ocamlfind-secondary 1.9.1
    ↻ recompile dune                2.9.1 [uses ocamlfind-secondary]
  ===== ↻ 1   ⊘ 1 =====
  

• but remains with remove -a:

  % opam remove ocamlfind-secondary --show -a
  The following actions would be faked:
    ⊘ remove dune                     2.9.1    [uses ocamlfind-secondary]
    ⊘ remove ocamlfind-secondary      1.9.1
    ⊘ remove ocamlfind                1.9.1
    ⊘ remove ocaml-secondary-compiler 4.08.1-1
  ===== ⊘ 4 =====
  

  dune was in my case part of the "root" packages and shouldn't have been removed.