"Upgrade is not possible because of conflicts" doesn't say where the conflict comes from

[RalfJung]

We have opam upgrade failing in a CI job with the following error:

[WARNING] Upgrade is not possible because of conflicts or packages that are no longer available:
    - Missing dependency:
    - coq (< 8.19~ | >= dev)
    not available because the package is pinned to version 8.19.dev
You may run "opam upgrade --fixup" to let opam fix the current state.

This error is not very helpful since it doesn't tell me where that dependency comes from. I am quite confused since I think we have bumped everything to the latest Coq version already, but I guess I missed one package somewhere.

That error message should say which is the package that depends on coq with this version constraint.

# opam config report
# opam-version         2.1.5 
# self-upgrade         no
# system               arch=x86_64 os=linux os-distribution=debian os-version=10
# solver               builtin-mccs+glpk
# install-criteria     -removed,-count[avoid-version,changed],-count[version-lag,request],-count[version-lag,changed],-count[missing-depexts,changed],-changed
# upgrade-criteria     -removed,-count[avoid-version,changed],-count[version-lag,solution],-count[missing-depexts,changed],-new
# jobs                 20
# repositories         4 (http), 1 (version-controlled) (default repo at 0286027b)
# pinned               6 (git), 1 (rsync), 2 (version)
# current-switch       default
# ocaml:native         true
# ocaml:native-tools   true
# ocaml:native-dynlink true
# ocaml:stubsdir       /builds/iris/examples/_opam/default/lib/ocaml/stublibs:/builds/iris/examples/_opam/default/lib/ocaml
# ocaml:preinstalled   false
# ocaml:compiler       4.14.0+options+flambda

[RalfJung]

Hm, looking at this again, this is just a warning... but opam does exit with a non-zero status code and the CI job quits right after this, and that's all the output I have. Here is the full CI log.

[rjbou]

From dev meeting: We need to change that warning into error, and check exit code for no upgrade vs partial upgrade.