[Rd] R CMD check warning with S3 method

[Winston Chang]

I'm getting an R CMD check warning with a package (call it package A)
that defines an S3 method but not the generic. The generic is defined
in another package (package B). Package A imports the S3 generic from
B. And there's one additional detail: the generic overrides a function
in the stats package.

I've created a minimal test package which reproduces the problem:
  https://github.com/wch/s3methodtest

In this case:
- the package imports dplyr, for the dplyr::filter S3 generic
- the package defines a S3 method filter.test
- it imports dplyr, which defines a filter S3 generic

The warning doesn't occur when package dplyr is in Depends instead of
Imports. It also doesn't occur if the method is for a generic that
does not override an existing function like stats::filter. For
example, if instead of filter.test, I define select.test
(dplyr::select is also an S3 generic), then there's no warning.

This warning seems incorrect. Is this a bug? I'm interested in
submitting the package to CRAN soon, so any advice on what to do is
appreciated.

Thanks,
-Winston