T7175.hs:8:4: error:
    Data constructor ‘G1C’ returns type ‘F Int’
      instead of an instance of its parent type ‘G1 a’
    In the definition of data constructor ‘G1C’
    In the data type declaration for ‘G1’

T7175.hs:11:4: error:
    Data constructor ‘G2C’ returns type ‘F Int’
      instead of an instance of its parent type ‘G2 a’
    In the definition of data constructor ‘G2C’
    In the data type declaration for ‘G2’