Bug#900018: FTBFS with latest cmdliner

[Mehdi Dogguy <mehdi@debian.org>]

Package: opam
Version: 1.2.2-6+b1
Severity: serious

opam fails to build from source using latest cmdliner which was uploaded
to Debian/Sid a few days ago:

File "client/opamArg.ml", line 384, characters 25-29:
Error: This expression has type
         ?docv:string ->
         (string -> ('a, [ `Msg of string ]) result) * 'a printer ->
         'a converter
       but an expression was expected of type
         'b converter = 'b parser * 'b printer
../OCamlMakefile:1076: recipe for target 'client/opamArg.cmo' failed

Full build log can be found here:

   https://buildd.debian.org/status/fetch.php?pkg=opam&arch=armel&ver=1.2.2-6%2Bb3&stamp=1526941860&raw=0

-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.16.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages opam depends on:
ii  build-essential  12.4
ii  curl             7.58.0-2
ii  libbz2-1.0       1.0.6-8.1
ii  libc6            2.27-3
ii  opam-docs        1.2.2-6
ii  tar              1.30+dfsg-2
ii  unzip            6.0-21
ii  wget             1.19.5-1
ii  zlib1g           1:1.2.11.dfsg-1

Versions of packages opam recommends:
ii  aspcud     1:1.9.4-1
pn  darcs      <none>
ii  git        1:2.17.0-1
pn  mercurial  <none>
pn  ocaml      <none>
ii  rsync      3.1.2-2.1

opam suggests no packages.

-- no debconf information