tcfail121.hs:13:9: error: [GHC-43085]
    • Overlapping instances for Foo [a] arising from a use of ‘op’
      Matching instance:
        instance [overlappable] Foo a => Foo [a]
          -- Defined at tcfail121.hs:9:31
      Potentially matching instance:
        instance [overlapping] Foo [Int] -- Defined at tcfail121.hs:10:30
      (The choice depends on the instantiation of ‘a’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • In the expression: op x
      In an equation for ‘foo’: foo x = op x