Pattern synonyms allow multiple type signatures but only use the first

GHC seems to only be using the first signature for pattern synonyms

GHC 8.0.2 allows this to compile:

{-# LANGUAGE PatternSynonyms #-}
module Pattern where

pattern Foo :: ()
pattern Foo :: Bool
pattern Foo = ()

but not this:

{-# LANGUAGE PatternSynonyms #-}
module Pattern where

pattern Foo :: Bool
pattern Foo :: ()
pattern Foo = ()

This compiles and ghci tells me the type of Foo is ():

{-# LANGUAGE PatternSynonyms #-}
module Pattern where

pattern Foo :: ()
pattern Foo :: Bool
pattern Foo <- _
  where Foo = undefined

This doesn't compile (as expected):

{-# LANGUAGE PatternSynonyms #-}
module Pattern where

pattern Foo :: ()
pattern Foo :: Bool Bool
pattern Foo = ()

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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