Opam suggests change to OCaml variant type on conflict

[craigfe]

On Opam version 2.1.0~alpha, when using a switch with ocaml-variants, opam seems to make no effort to preserve the type of variant installed. For instance,

in which opam suggests a switch from a +afl variant to a +statistical-memprof variant in addition to the necessary change to the base compiler version.

This presumably happens because the version of ocaml-variants is opaque to Opam, and so 4.07+afl and 4.07+statistical-memprof are equally valid solutions. My expectation is that users would want the type of OCaml compiler variant they have installed to be preserved if at all possible, using some sort of "invariant" along the lines of:

ocaml-variants = .*+afl

Since this isn't a valid Opam constraint, it's not possible to set it as a switch invariant. The solution here may be to change the way ocaml-variants works to avoid overloading the version string as it does (perhaps ocaml-variants-afl.4.09 rather than ocaml-variants.4.09+afl?).

[AltGr]

Your diagnostic is correct, the way these variants are handled is a bit of a hack in the opam-repository, and your result is the downside of it. We have better ways of specifying compiler options and variants in the works, but nothing fully designed yet.

In your case, you probably want to set the switch base (or invariant in 2.1) so that opam won't suggest changing its version at all.

[dra27]

This has been addressed in opam-repository with the new ocaml-option- packages!