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

3 files changed, 75 insertions, 0 deletions

diff --git a/testsuite/tests/warnings/should_compile/T11128b.hs b/testsuite/tests/warnings/should_compile/T11128b.hs
new file mode 100644
index 0000000000..2cca9a53e0
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T11128b.hs
@@ -0,0 +1,64 @@
+{-# LANGUAGE DeriveFunctor #-}
+{-# OPTIONS_GHC -Wnoncanonical-monadfail-instances  #-}
+
+-- | Test noncanonical-monadfail-instances warnings
+module T11128b where
+
+import Control.Applicative as A
+import Control.Monad as M
+import Control.Monad.Fail as MF
+
+----------------------------------------------------------------------------
+-- minimal definition
+
+data T0 a  = T0 a deriving Functor
+
+instance A.Applicative T0 where
+    pure   = T0
+    (<*>)  = M.ap
+
+instance M.Monad T0 where
+    (>>=)  = undefined
+
+instance MF.MonadFail T0 where
+    fail   = error "fail"
+
+----------------------------------------------------------------------------
+-- trigger all 2 warnings
+
+data T1 a  = T1 a deriving Functor
+
+instance A.Applicative T1 where
+    pure   = return
+    (<*>)  = M.ap
+    (*>)   = (M.>>)
+
+instance M.Monad T1 where
+    (>>=)  = undefined
+    return = T1
+    (>>)   = undefined
+    fail   = error "fail"
+
+instance MF.MonadFail T1 where
+    fail   = M.fail
+
+----------------------------------------------------------------------------
+-- backward compat canonical defintion
+
+data T2 a  = T2 a deriving Functor
+
+instance Applicative T2 where
+    pure   = T2
+    (<*>)  = ap
+    (*>)   = undefined
+
+instance M.Monad T2 where
+    (>>=)  = undefined
+    return = pure
+    (>>)   = (*>)
+    fail   = MF.fail
+
+instance MF.MonadFail T2 where
+    fail   = error "fail"
+
+----------------------------------------------------------------------------
diff --git a/testsuite/tests/warnings/should_compile/T11128b.stderr b/testsuite/tests/warnings/should_compile/T11128b.stderr
new file mode 100644
index 0000000000..57aa22beea
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/T11128b.stderr
@@ -0,0 +1,10 @@
+
+T11128b.hs:40:5: warning:
+    Noncanonical ‘fail’ definition detected
+    in the instance declaration for ‘Monad T1’.
+    Either remove definition for ‘fail’ or define as ‘fail = Control.Monad.Fail.fail’
+
+T11128b.hs:43:5: warning:
+    Noncanonical ‘fail = Control.Monad.fail’ definition detected
+    in the instance declaration for ‘MonadFail T1’.
+    Move definition from ‘Control.Monad.fail’ to ‘fail’
diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T
index a2b1860ba4..2e7132213c 100644
--- a/testsuite/tests/warnings/should_compile/all.T
+++ b/testsuite/tests/warnings/should_compile/all.T
@@ -7,6 +7,7 @@ test('T9230', normal, compile_without_flag('-fno-warn-tabs'), [''])
 test('T10908', normal, compile, [''])
 test('T11077', normal, compile, ['-fwarn-missing-exported-sigs'])
 test('T11128', normal, compile, [''])
+test('T11128b', normal, compile, [''])
 test('PluralS', normal, compile, [''])
 
 test('DeprU',