Implement `-Wnoncanonical-monadfail-instances` warning

The MonadFail proposal implemented so far via #10751 only warns about
missing `MonadFail` instances based on existence of failible pattern
matches in `do`-blocks.

However, based on the noncanonical Monad warnings implemented via #11150
we can provide a different mechanism for detecting missing `MonadFail`
instances quite cheaply. That is, by checking for canonical `fail` definitions.

In the case of `Monad`/`MonadFail`, we define the canonical implementation of
`fail` to be such that the soft-deprecated method shall (iff overridden) be
defined in terms of the non-deprecated method. Consequently, in case of
`MonadFail`, the `Monad(fail)` method shall be defined as alias of
the `MonadFail(fail)` method.

This allows us at some distant point in the future to remove `fail` from
the `Monad` class, while having GHC ignore/tolerate such literal canonical
method definitions.

Reviewed By: bgamari, RyanGlScott

Differential Revision: https://phabricator.haskell.org/D1838

1 files changed, 8 insertions, 0 deletions

diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index e7c93e1d7c..0c5260f434 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -197,6 +197,14 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Wno-noncanonical-monad-instances"
          }
+  , flag { flagName = "-Wnoncanonical-monadfail-instances"
+         , flagDescription =
+           "warn when ``Monad`` or ``MonadFail`` instances have "++
+           "noncanonical definitions of ``fail``."++
+           "See flag description in :ref:`options-sanity` for more details."
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-noncanonical-monadfail-instances"
+         }
   , flag { flagName = "-Wnoncanonical-monoid-instances"
          , flagDescription =
            "warn when ``Semigroup`` or ``Monoid`` instances have "++