Why does it not cause a warning that gamm4 uses internal function from mgcv?

I am puzzled by a fact that I noticed when reading the source code of the gamm4 package.

In two places, it imports internal functions from mgcv.

One place is in the gamm4.setup function (link to code here)

  G <- mgcv:::gam.setup(formula,pterms,
                 data=data,knots=knots,sp=NULL,
                 min.sp=NULL,H=NULL,absorb.cons=TRUE,sparse.cons=0,gamm.call=TRUE)

Another place is in the gamm4 function (link to code here):

  var.summary <- mgcv:::variable.summary(gp$pf,dl,nrow(mf)) ## summarize the input data

In the DESCRIPTION file, it lists mgcv under Depends (link to full DESCRIPTION file here):

Depends: R (>= 2.9.0), methods, Matrix, lme4 (>= 1.0), mgcv (>= 1.7-23)

Furthermore, in NAMESPACE it imports mgcv, although I don't think that's relevant for my question. Link to NAMESPACE here.

This use of internal functions from another package causes a warning if I do exactly the same thing in a package that I'm developing, leaving the following WARNING from R CMD check:

❯ checking dependencies in R code ... WARNING
  Unexported objects imported by ':::' calls:
    ‘mgcv:::gam.setup’ ‘mgcv:::variable.summary’
    See the note in ?`:::` about the use of this operator.
    Including base/recommended package(s):
    ‘mgcv’

However, gamm4 has no NOTEs or WARNINGs on CRAN, and I can confirm by downloading the source code and running R CMD check on it, that everything is fine for gamm4.

Now my question is:

What is it that I'm missing? What is the trick that the author of gamm4 does, so that using an internal function from another package causes no WARNING?

Note: I'm aware that using an internal still may not be good practice, e.g., as noted in help(":::"), so my question mainly regards why R CMD check accepts this.

Solution

Edit: thanks to @GaborCsardi's comment, I learnt that using ::: in a package doesn't raise a warning if you are also the maintainer of the package from which you use the function.

For example, if you're the maintainer of the package foo, you can use foo::: in another package foo2 without having this warning. Note that this should only work if you use the same email address in the Maintainer field in DESCRIPTION.

In your example, Simon Wood is the maintainer of both gamm4 and mgcv.