tc168.hs:17:1: error: [GHC-39999]
    • Could not deduce ‘C a1 (a2, b0)’
      from the context: C a1 (a2, b)
        bound by the inferred type for ‘g’:
                   forall {a1} {a2} {b}. C a1 (a2, b) => a1 -> a2
        at tc168.hs:17:1-16
      The type variable ‘b0’ is ambiguous
    • In the ambiguity check for the inferred type for ‘g’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the inferred type
        g :: forall {a1} {a2} {b}. C a1 (a2, b) => a1 -> a2