Encourage top-level `build-depends` when it occurs in all branches of a conditional

[andreasabel]

See e.g. jvranish/Lenses#3.
Currently hackage does not allow revision of conditionals, and it does not allow new build-depends.
In the situation (1)

build-depends: base
  , A
if cond
  build-depends: B >= 0.1 && < 0.4
else
  build-depends: B >= 0.2 && < 0.5

the constraints for B cannot be revised.
Thus, the better practice is to put a top-level B with extra constraints in the conditional (2):

build-depends: base
  , A
  , B >= 0.1 && < 0.5
if cond
  build-depends: B < 0.4
else
  build-depends: B >= 0.2

If cabal check would suggest a adding a top-level build-depends that appears in all branches, that would help with revisions on hackage.

Of course, this could also be fixed on the hackage side; in case it is fixed there by rejecting packages that could be rewritten from (1) to (2), cabal check should also warn about possible complaints by the hackage server.

[phadej]

constraints for B definitely can be revised. See eg. https://hackage.haskell.org/package/aeson-0.8.1.1/revisions/ which revised

  if flag(old-locale)
    build-depends: time < 1.5, old-locale
  else
    build-depends: time >= 1.5

to

  if flag(old-locale)
    build-depends: time >= 1.1.3 && < 1.5, old-locale
  else
    build-depends: time >= 1.5

---

I assume you use hackage-cli add-bound feature, which isn't smart enough to deal with these.

---

EDIT: doesn't seem to be a problem with lenses either (that's a preview of a possible revision):

[andreasabel]

Thanks for the swift feedback, @phadej!

I assume you use hackage-cli add-bound feature, which isn't smart enough to deal with these.

Mmh, weird, I cannot remember how I came to the conviction that conditional bounds cannot be revised.
Maybe I mixed this up with the fact that you cannot "add" a top-level dependency even if it is already mentioned in conditionals. I don't think I used hackage-cli add-bound ...

In the case of lenses, if there was a top-level build-depends: template-haskell, it could be equipped with < 2.17 rather than doing it in all of the branches. So maybe there is still some value to my suggestion, but certainly less need. It could be a warning or style suggestion (linting).