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{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeApplications #-}
module T21662 where
import GHC.TypeNats (Nat, KnownNat)
view_fn :: forall (n :: Nat). KnownNat n => Int -> Bool
view_fn i = i > 0
foo :: Int -> Int
foo (view_fn @12 -> True) = 0
foo (view_fn @12 -> False) = 0
-- The point is that the two view pattern functions "view_fn"
-- may get different uniques for the KnownNat 12 dictionary,
-- which leads to a spurious pattern match warning.
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