Spurious unused quantified type variable warning with ExistentialQuantification

When using an existentially quantified type variable that shadows another variable in a data declaration, -Wall produces more warnings than necessary:

$ ghci -XExistentialQuantification -Wall
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
λ> data Ex a = forall a. Ex a

<interactive>:2:20: Warning:
    Unused quantified type variable ‘a’
    In the definition of data constructor ‘Ex’

<interactive>:2:20: Warning:
    This binding for ‘a’ shadows the existing binding
      bound at <interactive>:2:9

The unused quantified type variable warning seems wrong, since it is being used (as the shadowing warning indicates). Curiously, this warning doesn't seem to appear when doing something similar with RankNTypes:

$ ghci -XRankNTypes -Wall
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
λ> data Ex a = Ex (forall a. a)

<interactive>:2:24: Warning:
    This binding for ‘a’ shadows the existing binding
      bound at <interactive>:2:9

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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changed weight to 5

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Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

added typechecker label

closed

I can't reproduce this anymore on GHC 8.1.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

mentioned in commit 1ee92293

Trac metadata

Trac field	Value
Test case	- → rename/should_compile/T10625

changed milestone to %8.0.1

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