This constraint already exists and work.  You can define `gem.add_dependency
'mail', '~> 2.6', '>= 2.6.4', '!= 2.7.0'`.

See
https://github.com/rubygems/rubygems/blob/89ce39a27611f7a791562cecf53bf2d773ae105f/lib/rubygems/requirement.rb#L7

On Sun, Feb 24, 2019 at 13:41 <skalee@gmail.com> wrote:

> Issue #15619 has been reported by skalee (Sebastian Skalacki).
>
> ----------------------------------------
> Feature #15619: Blacklist certain dependency versions
> https://bugs.ruby-lang.org/issues/15619
>
> * Author: skalee (Sebastian Skalacki)
> * Status: Open
> * Priority: Normal
> * Assignee:
> * Target version:
> ----------------------------------------
> # Abstract
>
> This feature request proposes introducing a new dependency constraint
> "!=", which will allow to blacklist a specific buggy version of some gem
> dependency without dropping support for older releases.
>
> # Background
>
> I am developing a gem which extends functionality of the Mail gem.  It
> works with Mail 2.6.4 onwards (the current latest is 2.7.1), therefore I'd
> normally define a dependency constraint as combination of "~> 2.6" AND ">=
> 2.6.4".
>
> However, there is one exception: Mail version 2.7.0 has some bug, which is
> fatal for my gem.  This bug has been fixed in 2.7.1.  I need to prevent
> users from using the buggy version of Mail with my gem.  Currently, I can
> do following:
>
> 1. Bump version constraint on Mail gem to "~> 2.7" AND ">= 2.7.1".
>
> 2. Release two separate gems (or versions), one with constraint "~>
> 2.6.4", and another with "~> 2.7" AND ">= 2.7.1".
>
> 3. Display a proper message in README and post-install step in order to
> inform users that they should care about Mail version themselves (e.g.
> constrain it in their gemfiles).
>
> 4. Perform a runtime check, and raise exception on incompatible Mail
> version.
>
> 5. Any reasonable combination of above.
>
> Option 1 seems to be the best.  It is easy and very straightforward, also
> it does not break Bundler's gem resolution.  However, it seems wrong to
> remove support for older versions only because single version of Mail is
> buggy.  What is more, such change to dependencies may be considered as a
> breaking one.  Option 2 adds an unnecessary maintenance burden, and feels
> odd in general.  Options 3 and 4 are also quite odd, as they seem to be an
> unnecessary complication, and may surprise users who have just upgraded my
> gem.
>
> Actually, what I would really want to achieve is to be able to define
> dependency constraint as "~> 2.6" AND ">= 2.6.4" BUT NOT "= 2.7.0".
>
> # Proposal
>
> For this reason, I propose introducing "!=" version constraints which
> exclude unwanted version explicitly.  For example, in my case I could write
> "~> 2.6" AND ">= 2.6.4" AND "!= 2.7.0".
>
>
>
> --
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