build issue with ambiguous generic `OPERATOR(==)` interface

Description

In this highly reduced test case from fpm-0.10.1.F90::

module fpm_toml
  type, abstract, public :: serializable_t
  contains
    procedure(is_equal), deferred :: serializable_is_same
    generic :: operator(==) => serializable_is_same
  end type
  abstract interface
    logical function is_equal(this,that)
      import serializable_t
      class(serializable_t), intent(in) :: this,that
    end function is_equal
  end interface
end module

module fpm_manifest_preprocess
  use fpm_toml
  type, extends(serializable_t) :: preprocess_config_t
  contains
    procedure :: serializable_is_same => preprocess_is_same
  end type
  interface operator(==)
    module procedure preprocess_is_same
  end interface
 contains
  logical function preprocess_is_same(this,that)
    class(preprocess_config_t), intent(in) :: this
    class(serializable_t), intent(in) :: that
    preprocess_is_same = .false.
  end function
end module

several compilers (flang-new, nagfor, XLF) produce an error message about an ambiguous generic interface for OPERATOR(==), and after some analysis I believe that they are right to do so. Although in this case it does turn out that the particular generic definitions would cause any reference to that generic procedure to resolve to the same specific procedure (preprocess_is_same), it is possible for other derived types to be defined (perhaps in clients of fpm_manifest_preprocess) that would be extensions of serializable_t or preprocess_config_t with distinct overrides of the deferred TBP serializable_is_same.

I suggest removing the non-type-bound generic interface for OPERATOR(==), if possible. It seems at best redundant, even if it did not lead to ambiguous resolution.

Expected Behaviour

Building without error.

Version of fpm

0.10.1

Platform and Architecture

n/a

Additional Information

No response

fortran-lang / fpm