Pattern synonym type signatures
Currently, the only way to specify a type signature for a pattern synonym is by adding type annotations to the right-hand side, like this:
{-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-}
pattern Single x = ([x] :: [Int])Bespoke syntax for adding type signatures for pattern synonyms would probably be useful.
Link to PatternSynonyms for pattern synonym signatures for specification.