[Bug 1791150] Re: cannot upgrade from previous stable version. Myabe I have " Unofficial software packages not provided by Ubuntu"

[Kin Mye]

Dear Brian,
Thank you very much for your support.
I don't know how you found it, but yes, this was the problem.
I need to install pa-purge and execute: 
ppa-purge  ubuntu-toolchain-r/test

I did not remember having that: many packages related with g++ where
downgraded and then the system upgrade to 18.04 worked perfectly.

Best,
Kin M.

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https://bugs.launchpad.net/bugs/1791150

Title:
  cannot upgrade from previous stable version.  Myabe I have "
  Unofficial software packages not provided by Ubuntu"

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

Bug description:
  I tried to upgrade from 16:04 to 18:04, but is says:

  Could not calculate 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.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: ubuntu-release-upgrader-core 1:16.04.25
  ProcVersionSignature: Ubuntu 4.15.0-33.36~16.04.1-generic 4.15.18
  Uname: Linux 4.15.0-33-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.18
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: Unity
  Date: Thu Sep  6 20:55:31 2018
  InstallationDate: Installed on 2016-12-22 (623 days ago)
  InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to xenial on 2018-09-06 (0 days ago)
  VarLogDistupgradeTermlog:

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