I've been working on a similar bug, possibly the same bug or subset of it? https://bugs.debian.org/972820 is one of its many names. I had made a reproducer minimal test case from my own experience, which I posted here https://salsa.debian.org/rpavlik/gcc-upgrade-testcase and, to get myself out of a jam, the corresponding "transitional" packages here: https://salsa.debian.org/rpavlik/gcc-10-compat (That repo also contains a list of a number of other places where that bug has been filed.)
I'm not sure if this is the same bug or not, but it seems related, at least, and of similar importance. As of my test a few minutes ago, my docker test case can now upgrade successfully, I'll see if I can find time to use my pre-upgrade |/var/lib/dpkg/status| to make a more complete test case to verify that one. I'll also look at the package changelog. Thanks for your work on the upgrade use case here. Ryan
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