LANGUAGE ConstraintKinds not needed to use ConstraintKinds?

[Eric Seidel]

On Sun, Feb 14, 2016, at 18:15, Evan Laforge wrote:
> Right, that sounds like a good idea to me, it's the same reason I
> added the synonym myself.
> 
> WRT not needing an extension I guess this is part of a general pattern
> where you don't need extensions to use code that uses extensions.  In
> this case though it seems a bit odd in that using the code that uses
> extensions also requires previously illegal syntax.  But as long as
> it's intentional it's fine by me.
> 
> But is the bit where the calling module needs FlexibleConstraints also
> intentional?

It looks like it is. According to [1], the context must have the form `C
a` unless FlexibleContexts is enabled.

[1]:
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/other-type-extensions.html#flexible-contexts