How to recover from "some outputs are unexpectedly invalid" error?

[l0b0]

Problem

I ran into this error message. There are no relevant messages near to it, so I have no idea how to recover from it. Basically, the error message would have to answer all of the following to be useful:

• Which paths?
• What does "invalid" mean in this context?
• Is the word "unexpectedly" even meaningful here? Is it possible for outputs to be expectedly invalid?
• Which exact steps do I need to follow to recover?

Proposal

Checklist

• [x] checked latest Nix manual (source)
• [x] checked open documentation issues and pull requests for possible duplicates

Priorities

Add :+1: to issues you find important.

[roberth]

This message comes from assertPathValidity, which is called from a couple of places during the build process. More context should be added to the error, so that at least the various usages of the function can be distinguished. Then we'll know where to look for a bug.