Why does GHC emit warning?

foo.hs

{-# LANGUAGE RecordWildCards #-}

data Foo = Foo
  { a :: Bool
  } 

run :: Foo -> Bool
run Foo{..}
  | a = a
  | not a = not a

main :: IO ()
main = return ()

output.txt

$ runghc -- -fwarn-incomplete-patterns -Wall exaustive.hs

exaustive.hs:10:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘run’: Patterns not matched: (Foo False)
   |
10 | run Foo{..}
   | ^^^^^^^^^^^...