I understand ghc complaining about b, but not about a.

No warnings when view patterns are not used.

{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS -Wmonomorphism-restriction #-}

module Foo where

{-

demo.hs:11:9: warning: [-Wmonomorphism-restriction]
    • The Monomorphism Restriction applies to the bindings for ‘a’, ‘b’
        Consider giving them a type signature
    • In the expression: let Foo a (round -> b) = f in a + b
      In an equation for ‘bar’:
          bar f = let Foo a (round -> b) = f in a + b
   |
11 |     let Foo a (round -> b) = f
   |         ^^^^^^^^^^^^^^^^^^^^^^
-} 

data Foo = Foo Int Double

bar :: Foo -> Int
bar f =
    let Foo a (round -> b) = f
     in a + b