ApplicativeDo: Pattern matching on a bind forces a Monad constraint

Pattern matching on a bind in 8.2.1 confuses ApplicativeDo, and causes the type to have a Monad constraint where a Functor constraint would suffice.

In GHC 8.0:

> :set -XApplicativeDo
> :t \x -> do { (a, _) <- x; pure (a+1) }
\x -> do { (a, _) <- x; pure (a+1) }
  :: (Num b, Functor f) => f (b, t) -> f b

In GHC 8.2:

> :set -XApplicativeDo
> :t \x -> do { (a, _) <- x; pure (a+1) }
\x -> do { (a, _) <- x; pure (a+1) }
  :: (Num b1, Monad m) => m (b1, b2) -> m b1

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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