ApplicativeDo seems to prevent the fail method from being used

I am not sure if this is intended, but I came across this issue when debugging a runtime exception. It seems like an incomplete pattern match in a do-block with ApplicativeDo enabled will not use the fail method.

If I have a module like this

{-# LANGUAGE ApplicativeDo #-}
{-# OPTIONS_ghc -ddump-simpl #-}
module M where

f :: Maybe (Maybe Int) -> Maybe Int -> Maybe Int
f mgs mid = do
  _ <- mid
  (Just moi) <- mgs
  pure (moi + 42)

main :: IO ()
main = print (f (Just Nothing) (Just 2))

This will result in a runtime exception:

Just *** Exception: repro.hs:(6,13)-(9,17): Non-exhaustive patterns in lambda

But if I remove the ApplicativeDo extension, this code works as normal, producing Nothing as the output.

On a theoretical level I understand why this is happening, but I still find it quite unexpected, especially since the documentation at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#things-to-watch-out-for claims that

Your code should just work as before when ApplicativeDo is enabled, provided you use conventional Applicative instances.

If this is not a defect in GHC itself, I'd say it is a defect in documentation in not pointing out this gotcha.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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