I think I have finally understood what's going on.

As Simon pointed out (that was the key insight) mapMaybeFP is referenced inside the RULE, but I'm working on the Lexer to remove old Haddock 0.x comments, and I suspect my patch broke the Lexer, so the GHC version on my branch is interpreting that rule as a normal comment, causing the aforementioned error.
Well, at least I know the problem is just mine and that my patch needs improvement :)

Thanks SPJ!
Alfredo

On Thursday, 12 June 2014, Alfredo Di Napoli <alfredo.dinapoli@gmail.com> wrote:
> Hi Simon,
> It's strange because I was not getting that before (say 2 weeks ago) and as far as I know I have not changed my build type (always been "quick").
> I'll try to see if playing around with -O will change things and report back.
> Thanks!
>
> Alfredo
> On 12/giu/2014, at 16:36, Simon Peyton Jones <simonpj@microsoft.com> wrote:
>
> Does not happen for me. Maybe you are compiling without –O (odd, but possible)? mapMaybeFB is mentioned only inside a RULE, so perhaps it’s regarded as un-referenced without –O?
>
>
>
> Simon
>
>
>
> From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Alfredo Di Napoli
> Sent: 12 June 2014 12:31
> To: ghc-devs@haskell.org
> Subject: Getting error about unused instances during "sh validate"
>
>
>
> Hello guys,
>
> am I the only one to get the following when I run "sh validate"?
>
>
> libraries/base/Data/Maybe.hs:139:1: Warning:
> Defined but not used: ‘mapMaybeFB’
>
> <no location info>:
> Failing due to -Werror.
> make[1]: *** [libraries/base/dist-install/doc/html/base/base.haddock] Error 1
> make[1]: *** Waiting for unfinished jobs....
> make: *** [all] Error 2
>
>
> I'm working on this ticket, but I can't see how that can be related:
>
> https://ghc.haskell.org/trac/ghc/ticket/8226
>
> Thanks!
> Alfredo