T23010_fail.hs-boot:7:1: error: [GHC-87543]
    Ambiguous occurrence ‘T23010_fail.fld’.
    It could refer to
       either the field ‘fld’ of record ‘A’,
              defined at T23010_fail.hs:6:16,
           or the field ‘fld’ of record ‘B’,
              defined at T23010_fail.hs:7:16.