T13871.hs:16:18: error: [GHC-88634]
    • Data constructor ‘MkFoo’ cannot be used here
        (it has an unpromotable context ‘(a ~ Int, b ~ Char)’)
    • In the first argument of ‘SFoo’, namely ‘MkFoo’
      In the type ‘SFoo MkFoo’
      In the definition of data constructor ‘SMkFoo’