custom-setup warning should only be suspicious

[ezyang]

If people want to support an old version of Cabal, they can't add the new setup build dependencies from #2515

So it should only be a suspicious warning, not a warning. This matters for GHC, since we hard error on all cabal check warnings.

[ezyang]

@hvr points out to me that if people follow this advice, their new package will instantly break all pre-7.10 users. Not great.

Warning in question is edsko@529912e

[hvr]

there's another problem if people start setting cabal-version: >= 1.23 to soon w/o also setting a relatively high base-lower bound: old cabal-install versions don't include the cabal-version constraint into the cabal-solving, and hence don't backtrack. This can cause cabal install foo to instantly break for all pre cabal-install-1.23 users as soon as foo starts publishing a version w/ cabal-version: >= 1.23

[23Skidoo]

/cc @edsko

[edsko]

So what's the conclusion here?

/pinging also @dcoutts

[dcoutts]

Right, build warning is too high here. It's at most dist suspicious.

As for the compat issue. Yes technically we don't need to require cabal-version: >= 1.23 for packages that use the new section. What we could do is make sure this works:

• if the new setup section exists, use it (in new cabal)
• old cabal should ignore the new section, with a warning. I think that's the behaviour, but have to double check.
• Have a check that if the package uses a custom setup, suggest (at dist suspicious level) using the new section type, but don't require cabal-version: >= 1.23. We can note in the check message that it is not necessary to use cabal-version: >= 1.23, however older cabal will not read the section so it may or may not work with old cabal (as it relies on the old behaviour of using what's in the ambient environment).
• if cabal-version: >= 1.23 however, and a package still uses a custom setup without using the new section, make that a build warning (or at minimum dist inexcusable)

[edsko]

Ok, @dcoutts has told me he will fix this :)

[ezyang]

This is still open because the committed change was only a partial fix.

[ezyang]

Punting.