I wonder if like TypeError we could have TypeWarning? Then you could
have something like

instance TypeWarning "Please use Fooble instead" => Show Foo where

which would compile, but produce a warning whenever the `Show Foo`
constraint was solved?

On Mon, Nov 25, 2019 at 4:28 PM Andreas Abel <andreas.abel@ifi.lmu.de> wrote:
>
> Derailing...
> Incidently, I recently asked for a "instance warning" feature in GHC:
>
>    https://gitlab.haskell.org/ghc/ghc/issues/17485
>
> (In my case I could go with a TypeError constraint instead.)
>
> Maybe instance warnings could be a useful feature.  It could be
> implemented along the TypeError mechanism.
>
> On 2019-11-24 18:35, Henning Thielemann wrote:
> >
> > On Sun, 24 Nov 2019, Dan Burton wrote:
> >
> >> We're talking about an instance in `base`, so if there's no other way,
> >> it could at least be a flag akin to
> >> -Wmissing-monadfail-instances that gets included in -Wcompat.
> >> But relatedly: should there be a way to deprecate instances? I would
> >> say yes.
> >
> > Sounds like another application of instance warnings that we already
> > talked about:
> >     https://gitlab.haskell.org/ghc/ghc/issues/11796
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