[GHC] #11160: New exhaustiveness checker breaks ghcirun004

[Ben Gamari]

Simon Peyton Jones <simonpj at microsoft.com> writes:

> No let’s not revert.  But:
>
> ·         Please put a list of the tickets (there are several) on the wiki page (where is the wiki page?)
>
> o   perf/compiler/T783
>
> o   #11163: perf/compiler/T5642
>
> o
>
> ·         Let’s have a flag to skip overlap testing so that there is always a workaround
>
Indeed, given that there appear to be relatively few (and, moreover, fairly pathological)
programs that send things astray this should be a fine workaround for 8.0. While
you introduce the flag be sure to add a mention in the documentation:

  * Update the note in `docs/user_guide/bugs.rst` to reflect the current
    state of the pattern match check.

  * Add a point to `bugs.rst` to describe the performance issue and the
    workaround. Describe, if you can, some heuristics that users might
    use to determine whether skipping overlap testing might be necessary
    for their program.

  * A note in `docs/user_guide/using-warnings.rst` describing the
    new flag, linking to the description of the performance issue in
    `bugs.rst`
    
  * Add a description of the flag to `utils/mkUserGuidePart/Options/Warnings.hs`

Also, perhaps we could handle future work via the usual Phabricator
route? While we sometimes do deviate from Phabricator for larger patches
like the original exhaustiveness checker merge, typically all other
patches are processed through Phabricator, which offers the advantage
that we can easily see in advance whether things will break.


> From: George Karachalias [mailto:george.karachalias at gmail.com]
> Sent: 04 December 2015 09:41
> To: Simon Peyton Jones <simonpj at microsoft.com>
> Subject: Re: [GHC] #11160: New exhaustiveness checker breaks ghcirun004
>
> I feel terrible since we are really close to the freeze and I think I am stalling everyone. 

Don't fret too much, it happens. Moreover, I deserve a large portion of
the blame as I neglected to mention the existence of our validation
script. Regardless things are pretty much back to normal at this point;
life goes on...

Cheers,

- Ben
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