Overlapping/Overlappable doesn't seem to do anything?

An example

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

class Foo a b where
    foo :: a -> b

instance {-# OVERLAPPABLE #-} Show a => Foo a String where
    foo = show

instance {-# OVERLAPPING #-} Read b => Foo String b where
    foo = read

bar :: String -> String
bar = foo

files to compile with

    • Overlapping instances for Foo String String
        arising from a use of ‘foo’
      Matching instances:
        instance [overlappable] Show a => Foo a String
          -- Defined at Incoh.hs:8:31
        instance [overlapping] Read b => Foo String b
          -- Defined at Incoh.hs:11:30
    • In the expression: foo
      In an equation for ‘bar’: bar = foo
   |
15 | bar = foo
   |     

but other one is overlapping, and other one is overlappable, shouldn't the overlapping be picked?

When I read the documentation, the problem is that to dismiss other instance either one have to be strictly more specific than other, but the error message doesn't remind the user about that requirement.

The error is therefore confusing.