T4921.hs:10:9: error:
    • Ambiguous type variables ‘a0’, ‘b1’ arising from a use of ‘f’
      prevents the constraint ‘(C a0 b1)’ from being solved.
      Relevant bindings include x :: a0 (bound at T4921.hs:10:1)
      Probable fix: use a type annotation to specify what ‘a0’, ‘b1’ should be.
      These potential instance exist:
        instance C Int Char -- Defined at T4921.hs:7:10
    • In the first argument of ‘fst’, namely ‘f’
      In the expression: fst f
      In an equation for ‘x’: x = fst f

T4921.hs:12:9: error:
    • Ambiguous type variable ‘b0’ arising from a use of ‘f’
      prevents the constraint ‘(C Int b0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘b0’ should be.
      These potential instance exist:
        instance C Int Char -- Defined at T4921.hs:7:10
    • In the first argument of ‘fst’, namely ‘f’
      In the expression: fst f :: Int
      In an equation for ‘y’: y = fst f :: Int