Hey Herbert,
I ran into this very problem early on. I'll use the opportunity to shamelessly plug an older package of mine probably only few know about which solved this very problem in mission critical systems for me while avoiding to have to enumerate O(n^2) transitions explicitly:
http://hackage.haskell.org/package/int-cast-0.1.2.0/docs/Data-IntCast.html This is very useful, thanks for that.
Tbh, given how ubiquitous the use of `fromIntegral` is throughout the Haskell ecosystem, having 'fromIntegral' suddely emit warnings when `-Wall` is active is not realistic IMHO.
Here I disagree. It is "realistic" because we can just do it, if we want. The questions more if we want to. I'd say yes, because: Deprecation-marking does not break any code, it is the softest way to make a transition. The Java community has this figured out: Deprecations are common, they don't break the build, they don't change behaviour, downstream libraries adapt quickly to them, and the whole ecosystem can advance to a more solid foundation gracefully. The way you phrase it sounds as if you'd expect a large backlash from your users if you emitted these extra warnings. But I think the opposite can be expected: Haskell users would love if -Wall pointed out *more* ways in which their code may break. We use -Wall because we *want* to be warned of slumbering bugs. I think your users would cheer at you, not complain. (Even the C people regularly cheer as new warnings make it into -Wall of gcc and clang.)