[2.1.0~alpha] Solver failures are too verbose

[kit-ty-kate]

Using opam master (3def1a6), when opam cannot find a solution to install a package, the breakdown of why the package can't be installed became too verbose compared to previous opam versions.
For instance when trying to install pgocaml.4.2 on ocaml-variants.4.11.0+trunk, the error message used to be the following:

The following dependencies couldn't be met:
  - pgocaml -> ppx_deriving >= 4.0 -> ocaml < 4.11.0
      base of this switch (use `--unlock-base' to force)

No solution found, exiting

With opam 2.1.0~alpha, the error message became:

The following dependencies couldn't be met:
  - pgocaml -> ppx_deriving >= 4.0 -> ocaml < 4.11.0 -> ocaml-base-compiler (!= 3.07+1 & != 3.09.3 & != 3.11.0 & != 4.02.1 & != 4.07.1)
      conflict with the base packages of this switch
  - pgocaml -> ppx_deriving >= 4.0 -> ocaml < 4.11.0 -> ocaml-variants < 4.10.2~
      incompatible with the switch invariant ["ocaml-variants" {= "4.11.0+trunk"}] (use `--update-invariant' to force)
  - ocaml-variants -> ocaml = 4.11.0
Your request can't be satisfied:
  - No available version of ocaml satisfies the constraints
  - No available version of ocaml-variants satisfies the constraints
  - ocaml-variants (!= 4.00.0+debug-runtime & < 4.00.1+annot | >= 4.00.1+french & != 4.01.0+PIC & != 4.01.0+open-types & < 4.02.1+32bit | = 4.02.1+modular-implicits | >= 4.02.1+musl+static & != 4.02.3+buckle-1 & < 4.03.0+32bit |
>= 4.03.0+beta1+flambda & != 4.04.0+32bit & < 4.04.0+afl | = 4.04.0+beta1+flambda | >= 4.04.0+beta2+flambda & < 4.04.0+fPIC | >= 4.04.0+fp & < 4.04.0+spacetime | >= 4.04.1+32bit & != 4.04.1+fp & != 4.04.2+32bit & != 4.04.2+fp &
!= 4.04.2+safe-string & != 4.05.0+32bit & != 4.05.0+beta1+flambda & != 4.05.0+lto & != 4.05.0+musl+static+flambda & < 4.05.0+rc1+flambda | >= 4.05.0+spacetime & != 4.05.0+trunk+flambda & < 4.06.0+32bit | >= 4.06.0+beta1 & !=
4.06.0+beta2+default-unsafe-string & < 4.06.0+beta2+fp+flambda | = 4.06.0+force-safe-string | = 4.06.0+rc1+afl | >= 4.06.0+rc1+fp & < 4.06.1+bytecode-only | >= 4.06.1+flambda & != 4.06.1+fp+flambda & !=
4.06.1+musl+static+flambda & != 4.06.1+rc1+force-safe-string & != 4.06.1+rc1+fp+flambda & != 4.06.1+rc2+default-unsafe-string & != 4.06.1+rc2+fp & != 4.06.1+trunk+afl & != 4.06.1+trunk+force-safe-string & < 4.07.0+32bit | =
4.07.0+beta2 | >= 4.07.0+beta2+default-unsafe-string & != 4.07.0+bytecode-only & != 4.07.0+rc1 & != 4.07.0+rc1+fp+flambda & != 4.07.0+rc2+fp+flambda & != 4.07.0+trunk+afl & != 4.07.1+afl & != 4.07.1+default-unsafe-string & <
4.07.1+force-safe-string | >= 4.07.1+musl+flambda & < 4.07.1+rc1+32bit | >= 4.07.1+rc1+default-unsafe-string & != 4.07.1+spacetime & < 4.08.0+beta1 | >= 4.08.0+beta1+flambda & < 4.08.0+beta1+fp+flambda | >= 4.08.0+beta2+afl & !=
4.08.0+beta3+afl & < 4.08.0+beta3+fp | >= 4.08.0+bytecode-only & < 4.08.0+flambda+no-flat-float-array | >= 4.08.0+fp & != 4.08.0+rc1+fp+flambda & != 4.08.0+rc2+flambda & < 4.08.1+bytecode-only | = 4.08.1+flambda | >=
4.08.1+fp+flambda & != 4.08.1+rc1+afl & != 4.08.1+rc2+force-safe-string & < 4.08.1+rc3+flambda | = 4.08.1+rc3+fp | >= 4.08.1+spacetime & < 4.08.1+trunk+force-safe-string | >= 4.08.1+trunk+fp+flambda & != 4.09.0+beta1+default-
unsafe-string & != 4.09.0+beta1+fp & < 4.09.0+beta2 | >= 4.09.0+beta2+default-unsafe-string & != 4.09.0+beta2+fp & < 4.09.0+default-unsafe-string | = 4.09.0+fp | = 4.09.0+musl+flambda | >= 4.09.0+spacetime & < 4.09.1+afl+flambda | =
4.09.1+flambda+no-flat-float-array | >= 4.09.1+fp & != 4.09.1+spacetime & < 4.10.0+32bit | = 4.11.0+trunk) is in conflict with ocaml-base-compiler

No solution found, exiting

I rely on these messages a lot in my work with the OCaml Software Foundation on getting the OCaml ecosystem ready for the next OCaml release and I find the new message rather unpleasant to work with. Is there a way to get the old behaviour back?

[AltGr]

Yup, sorry about this, we are aware of the regression (due to switch-invariants) and have already spent some time to improve them, but not completely there yet ; certainly expect them to be fixed before the release.