Panic with pattern synonym

[goldfirere]

Not that I expected it to work...

{-# LANGUAGE DeriveDataTypeable, TypeFamilies, TemplateHaskell, DataKinds,
             PolyKinds, GADTs, RankNTypes, MultiParamTypeClasses,
             FlexibleInstances, UndecidableInstances, CPP,
             FunctionalDependencies, StandaloneDeriving,
             TypeOperators, ScopedTypeVariables, NoMonomorphismRestriction,
             MonadComprehensions, DeriveGeneric, FlexibleContexts,
             GeneralizedNewtypeDeriving, ConstraintKinds,
             LambdaCase, ViewPatterns, AllowAmbiguousTypes,
             DefaultSignatures, -- ImpredicativeTypes,
             ImplicitParams, MagicHash, UnboxedTuples, RoleAnnotations,
             ExtendedDefaultRules, PatternSynonyms, EmptyCase,
             BangPatterns, InstanceSigs,
             DeriveFunctor, Arrows -- , PartialTypeSignatures
 #-}

module Scratch where

data T a where
  MkT :: (Show a, Show b) => a -> b -> T a

pattern P :: (Show a, Show b) => (Eq a, Num a) => b -> T a
pattern P x = MkT 3 x