Pattern synonym fails with (Exp Bool) but works with (t ~ Bool => Exp t)

The following compiles just fine:

data Exp a where
  B :: Bool -> Exp Bool

pattern Tru = B True

pr :: Exp a -> String
pr Tru = "true"

but add a signature to Tru…

pattern Tru :: Exp Bool
pattern Tru = B True

…and it fails to compile

% ghci -ignore-dot-ghci /tmp/Bug.hs
GHCi, version 7.10.0.20150316: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /tmp/Bug.hs, interpreted )

/tmp/Bug.hs:11:4:
    Couldn't match type ‘a’ with ‘Bool’
      ‘a’ is a rigid type variable bound by
          the type signature for pr :: Exp a -> String at /tmp/Bug.hs:10:7
    Expected type: Exp a
      Actual type: Exp Bool
    Relevant bindings include
      pr :: Exp a -> String (bound at /tmp/Bug.hs:11:1)
    In the pattern: Tru
    In an equation for ‘pr’: pr Tru = "true"
Failed, modules loaded: none.

Checking the inferred type of pattern Tru = B True I get

ghci> :i Tru
pattern Tru :: t ~ Bool => Exp t

which works as expected

pattern Tru :: t ~ Bool => Exp t
pattern Tru = B True

pr :: Exp a -> String
pr Tru = "true"

Trac metadata

Trac field	Value
Version	7.10.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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