records-req.hs

{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE PatternSynonyms, GADTs, ViewPatterns #-}

-- Pattern synonyms

module ShouldCompile where

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

f :: (Show a) => a -> Bool
f = undefined

pattern P{x} <- MkT (f -> True) x