Cannot pacify -Wmonomorphism-restriction with nested pattern bindings

I originally observed this issue in the parsers library. Here's a minimal example:

{-# LANGUAGE ScopedTypeVariables #-}
{-# OPTIONS_GHC -Wmonomorphism-restriction #-}
module Bug where

class Monad m => C m where
  c :: (m Char, m Char)

foo :: forall m. C m => m Char
foo = bar >> baz
  where
    (bar, baz) = c

If you compile this with a GHC that supports -Wmonomorphism-restriction (GHC 8.0.1 or later), it'll rightfully give this warning:

[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:11:5: warning: [-Wmonomorphism-restriction]
    • The Monomorphism Restriction applies to the bindings for ‘bar’,
                                                               ‘baz’
        Consider giving a type signature for these binders
    • In an equation for ‘foo’:
          foo
            = bar >> baz
            where
                (bar, baz) = c
   |
11 |     (bar, baz) = c
   |     ^^^^^^^^^^^^^^

Naturally, I tried to squelch this warning by adding type signatures for bar and baz:

foo :: forall m. C m => m Char
foo = bar >> baz
  where
    bar, baz :: m Char
    (bar, baz) = c

But GHC //still// warns!

[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:12:5: warning: [-Wmonomorphism-restriction]
    • The Monomorphism Restriction applies to the bindings for ‘bar’,
                                                               ‘baz’
        Consider giving a type signature for these binders
    • In an equation for ‘foo’:
          foo
            = bar >> baz
            where
                bar, baz :: m Char
                (bar, baz) = c
   |
12 |     (bar, baz) = c
   |     ^^^^^^^^^^^^^^

And to make things even more absurd, GHC points out a chunk of code which //has// type signatures :)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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