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Why do I have this error message : 

> vegan::reorder.hclust Error: 'reorder.hclust' is not an exported object from 'namespace:vegan'

While this S3 method is well available. For example if I type help(reorder.hclust, package = "vegan") I obtain the intended help page and vegan:::reorder.hclust displays the source code of the function on the console...
Also the NAMESPACE file of my vegan installation contains S3method(reorder, hclust).

I would like to use this function in another package were I need to import it or use vegan::reorder.hclust

> sessionInfo() R version 3.4.3 (2017-11-30) Platform: x86_64-pc-linux-gnu (64-bit) Running under: Ubuntu 16.04.4 LTS Matrix products: default BLAS: /usr/lib/libblas/libblas.so.3.6.0 LAPACK: /usr/lib/lapack/liblapack.so.3.6.0 locale: [1] LC_CTYPE=fr_BE.UTF-8 LC_NUMERIC=C LC_TIME=fr_BE.UTF-8 [4] LC_COLLATE=fr_BE.UTF-8 LC_MONETARY=fr_BE.UTF-8 LC_MESSAGES=fr_BE.UTF-8 [7] LC_PAPER=fr_BE.UTF-8 LC_NAME=C LC_ADDRESS=C [10] LC_TELEPHONE=C LC_MEASUREMENT=fr_BE.UTF-8 LC_IDENTIFICATION=C attached base packages: [1] stats graphics grDevices utils datasets methods base loaded via a namespace (and not attached): [1] MASS_7.3-49 compiler_3.4.3 Matrix_1.2-11 parallel_3.4.3 tools_3.4.3 [6] mgcv_1.8-23 yaml_2.1.18 nlme_3.1-131.1 grid_3.4.3 permute_0.9-4 [11] vegan_2.4-6 cluster_2.0.6 lattice_0.20-35
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You get that error because vegan doesn't export that function. It can give help for things that aren't exported. Using three colons vegan:::reorder.hclust displays internal functions that are not exported; normally you should avoid using those.

However, when the vegan package is loaded, its reorder.hclust function will be added to the methods table for reorder. So you just need to make sure that it is loaded, and then if hc is an hclust object, reorder(hc) will call the reorder.hclust method. You can do this by putting requireNamespace("vegan") into your code, or importing something from vegan in your NAMESPACE file.

If there are two reorder.hclust methods (defined by different packages that are both loaded), then I don't think there's an easy way for you to specify the vegan one other than using vegan:::reorder.hclust, which CRAN will object to. You would need to ask the vegan maintainer to export their function so you could access it using the legal vegan::reorder.hclust, or copy the code into your own package, or some other inconvenient approach.

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utils::getFromNamespace("reorder.hclust", "vegan") is one approach to use the function and pass CRAN test.
The docs warn against using getFromNamespace: "should not be used in production code". So CRAN may add this to its tests later.
Thanks for the answer ! Something I don't understand : isn't S3method(reorder, hclust) the way to export it from vegan (this is in the NAMESPACE file of the package) ?
To summarize : 3 solutions for the moment : 1) contact the vegan developer to ask him to export the function (but how if S3method(reorder, hclust) is not the way to do that ?), 2) copy the code in my package (not very nice - this is what I do for the moment) 3) according to the answer in the similar question pointed by @RolandASc : add Depends: vegan in the DESCRIPTION of my package to force this package to be attached (not very nice either). And maybe also ( use utils::getFromNamespace("reorder.hclust", "vegan") but this does not seem to be recommended.
Just two followups: the double dot notation does not cause the package to become attached, only loaded. It only works on functions exported from the package. So vegan::vegdist would trigger loading of the package if it is installed. Similarly, importFrom("vegan", "vegdist") would work to get it loaded, but you can't import something like reorder.hclust because it was not exported.
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