T20542.hs:17:7: error: [GHC-43085]
    • Overlapping instances for Foo String String
        arising from a use of ‘foo’
      Matching instances:
        instance [overlappable] Show a => Foo a String
          -- Defined at T20542.hs:10:31
        instance [overlapping] Read b => Foo String b
          -- Defined at T20542.hs:13:30
      An overlapping instance can only be chosen when it is strictly more specific.
      The first instance that follows overlaps the second, but is not more specific than it:
        instance [overlapping] Read b => Foo String b
        instance [overlappable] Show a => Foo a String
    • In the expression: foo
      In an equation for ‘bar’: bar = foo