Allow visible type application in patterns

EDIT: Though this ticket predates the proposal, it is a good place to track its development: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0126-type-applications-in-patterns.rst

Constructors (and pattern synonyms) when treated as expressions may be applied to types:

{-# LANGUAGE TypeApplications #-}

Nothing     :: Maybe a
Nothing @() :: Maybe ()

pattern Pair :: a -> a -> (a, a)
pattern Pair x y = (x, y)

Pair      :: a   -> a   -> (a, a)
Pair @Int :: Int -> Int -> (Int, Int)

But using them in patterns won't parse:

-- parse error in pattern: @Int
maybeToList :: Maybe Int -> [Int]
maybeToList (Nothing @Int) = []
maybeToList (Just @Int x)  = [x]

-- parse error in pattern: @Int
add :: (Int, Int) -> Int
add (Pair @Int x y) = x + y

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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