Error when using empty record update on binary pattern synonym

Using an empty record update with patterns works here

pattern Singleton a = [a]

isSingleton :: [a] -> Bool
isSingleton Singleton{} = True
isSingleton _           = False

but fails in the follow example (there may exist a more minimal example)

pattern P a b = Just (a, b)
foo P{} = True

which outputs (GHC 7.8.2)

$ ghci -ignore-dot-ghci tmp.g2WZtTUkhJ.hs
GHCi, version 7.8.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( tmp.g2WZtTUkhJ.hs, interpreted )
Var/Type length mismatch: 
    [t{tv aIW} [sk]]
    []
Ok, modules loaded: Main.
*Main> 

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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