use template to make return type match input type, so we don't need to
cast at the call sites

Change-Id: I1e65f362c67f74c9a230cdbc1db12545b28eb499
Reviewed-on: https://gerrit.libreoffice.org/71216
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>