ApplicativeDo return case doesn't handle lets

We ran into this bug in production, this is a simplified reproduction. The following program will not type check, requiring an unnecessary Monad instance:

{-# LANGUAGE ApplicativeDo, GeneralizedNewtypeDeriving #-}

import Data.Functor.Identity
import Data.Monoid

newtype A x = A (Identity x) deriving (Functor, Applicative)

shouldWork :: A ()
shouldWork = do
  a <- pure ()
  b <- pure ()
  let ab = a <> b
  return ab

pepe:~/code/snippets$ ghci ApplicativeDoBug.hs 
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( ApplicativeDoBug.hs, interpreted )

ApplicativeDoBug.hs:10:14: error:
    • No instance for (Monad A) arising from a do statement
    • In the expression:
        do { a <- pure ();
             b <- pure ();
             let ab = a <> b;
             return ab }
      In an equation for ‘shouldWork’:
          shouldWork
            = do { a <- pure ();
                   b <- pure ();
                   let ab = ...;
                   return ab }
Failed, modules loaded: none.

There is a simple workaround, which worked for us in production:

workaround :: A ()
workaround = do
  a <- pure ()
  b <- pure ()
  return $
    let ab = a <> b
    in ab

I asked in #ghc and it seems this is not yet fixed in HEAD.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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