[Bug 1787720] Re: Release upgrade fails (16.04 to 18.04.1)

[Alec Edgington]

Thanks. I may have got myself into more of a mess by proceeding with the
upgrade after installing libgfortran4. That allowed it to proceed, but I
still had mysterious broken packages afterwards, even after ppa-
purge'ing ubuntu-toolchain-r/test. The eventual solution was to remove
gcc-7-base, along with everything that depends on it (gcc etc), and then
reinstall gcc etc.

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https://bugs.launchpad.net/bugs/1787720

Title:
  Release upgrade fails (16.04 to 18.04.1)

Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid

Bug description:
  After being prompted by Software Updater to upgrade to 18.04.1 (from
  16.04), I clicked on 'Upgrade...' and followed the prompts. At the end
  of the 'Setting new software channels' stage, a window appeared
  containing this message:

  """
  Could not determine the upgrade

  An unresolvable problem occurred while calculating the upgrade.

   This can be caused by:
   * Upgrading to a pre-release version of Ubuntu
   * Running the current pre-release version of Ubuntu
   * Unofficial software packages not provided by Ubuntu

  If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.
  """

  Evidently neither of the first two bullets applies. It's not clear
  what the the third bullet encompasses, but I don't think I have any
  'unofficial' software that would be expected to interfere with the
  upgrade. ()

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: ubuntu-release-upgrader-core 1:16.04.25
  ProcVersionSignature: Ubuntu 4.4.0-133.159-generic 4.4.134
  Uname: Linux 4.4.0-133-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.18
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: Unity
  Date: Sat Aug 18 12:59:41 2018
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2016-04-22 (847 days ago)
  InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to xenial on 2018-08-18 (0 days ago)
  VarLogDistupgradeTermlog:

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