Reconsidering -Wall and -Wcompat

[Herbert Valerio Riedel]

On 2016-02-15 at 11:33:09 +0100, Boespflug, Mathieu wrote:

[...]

> * include -Wcompat and -Wall, but make it "magic" so that -Wcompat
> never causes a build to fail even when users set -Wall -Werror.

Tbh, I don't like the "magic" part at all. In fact, I currently rely on
`-Wcompat -Werror` triggering an error in my builds.

To address the concern more generally is what

  https://ghc.haskell.org/trac/ghc/ticket/11219

aims to, but without any magic. Then you'd say

  -Wall -Werror -Wno-error=compat

if -Wcompat implied by -Wall. But we ran out of time for GHC 8.0, so
#11219 will have to wait till GHC 8.2.