Hi Tobias,
A 1.1.4.0 build-depends: B ==2.5.* C ==3.7.* (overspecified) B 2.5.3.0 build-depends: C ==3.* (underspecified) C 3.7.1.0
Everything works nice until C-3.8.0.0 appears with incompatible changes that break B, but not A.
Now both A and B have to update their dependencies and we have now:
A 1.1.5.0 build-depends: B ==2.5.* C >=3.7 && <3.9 B 2.5.4.0 build-depends: C >=3 && <3.8 C 3.8.0.0
And now the following combination is still valid: A 1.1.5.0 B 2.5.3.0 (old version) C 3.8.0.0 Bang!
thank you for contributing this insightful example. When such a situation has arisen in the past, it's my experience that the author of B typically releases an update to fix the issue with the latest version of C: B 2.5.4.0 build-depends: C >= 3.8 So that particular conflict does hardly ever occur in practice. Note that package A would build just fine after that update of B -- if the author of A hadn't overspecified its dependencies. As it is, however, a new version of A has to released that changes no code, but only the Cabal file. Take care, Peter