cabal-version is not set properly when using ^>= bounds

[quasicomputational]

When ^>= is used, cabal-version needs to be at least 2.0. Hpack doesn't check for this, though.

[sol]

A PR would be more than welcome.

[tfausak]

For what it's worth, Cabal itself doesn't check for this either. If you run cabal check against a package description with ^>= bounds and cabal-version: >= 1.10, it will succeed.

Edited to add: Nevermind. I was wrong. cabal check correctly notices this problem.

[sol]

What we can do is parse ^>= and expand it as per the semantics given at https://www.haskell.org/cabal/users-guide/developing-packages.html#pkg-field-build-depends.

Independent from that, I think we want some syntactic sugar for version ranges, where he lower bound is inclusive and the upper bound is exclusive. Stuff like base >= 4.3.0.0 && < 5 is so common that I think it deserves dedicated support. I'm just not sure what the syntax would be, maybe

foo: x ~ y  # this desugars to ">= x && < 5"

If anybody has input on this, please comment!

[tfausak]

Adding custom operators reminds me of #91. I don't think it's a bad idea per se, but it could get tricky if Cabal ends up using the same operator for something else. I think it would be better to lobby for the change in Cabal, then add it to hpack for users that aren't on the latest Cabal.