Re #2:
> The version of libstdc++6 in vivid comes from gcc-4.9 source. There has been 
> an SRU of gccgo-4.9 to trusty, which requires it to build a gcc-4.9-base 
> package, whose version number is higher in trusty than in vivid; and 
> libstdc++6 requires the matching version of gcc-4.9-base. This prevents 
> upgrading anything that depends on libstdc++6.
> 
> This problem is specific to upgrades between trusty and vivid; libstdc++6 in 
> wily is built from gcc 5 so depends on gcc-5-base instead of gcc-4.9-base, 
> removing the problem.

>From this, I speculated that the obstruction in the upgrade path *might*
be cleared by manually up- (or rather down-) grading gcc-4.9-base from
the 4.9.3 version used in 14.04/trusty to the 4.9.2 version used in
15.04/vivid.

So I downloaded the package
(http://packages.ubuntu.com/vivid/gcc-4.9-base), manually installed it
with dpkg, and attempted to run do-release-upgrade again.

I haven't actually upgraded yet, but the error is gone now. The upgrade
path can be calculated successfully:

> 38 packages are going to be removed. 353 new packages are going to be 
> installed. 2364 packages are going to be upgraded.

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https://bugs.launchpad.net/bugs/1534374

Title:
  unable to upgrade to 15.04 due to libstdc++6 SRU

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