Parse error leads to exception with UnboxedTuples

[NeilMitchell]

Parse error and then an exception thrown while pretty printing the parse error! All on HSE 1.9.4.

Language.Haskell.Exts> parseDeclWithMode defaultParseMode{extensions=[UnboxedTuples]} "(##) :: m"
ParseFailed (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 1, srcColumn = 10}) "Left-hand side of type signature is not a variable: *** Exception: src\Language\Haskell\Exts\Pretty.hs:(1089,0)-(1094,21): Non-exhaustive patterns in function specialName