Because when you comment out neg, essentially you are just using the inherited method diff. “A Fortran extended type inherits all of the type parameters, components and nonoverridden, nonfinal procedure bindings from its parent type” (Extensible derived types (Fortran 2003) - IBM Documentation). But when you do generic :: operator(-) => diff, neg, the generic binding is not inherited and you will have to define your operator(-) for the extended type as well. I suggest you read @FortranFan’s answer in Custom operators in extended type - Help - Fortran Discourse (fortran-lang.discourse.group).