Found the following significant warnings:
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:224:21: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:226:21: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:396:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:397:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:398:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:399:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:400:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:401:51: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:403:78: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:405:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:406:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:407:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:408:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:409:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:410:52: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
  ../inst/dlib-19.20/dlib/array2d/../geometry/../matrix/../general_hash/murmur_hash3.h:411:51: warning: use of the 'fallthrough' attribute is a C++17 extension [-Wc++17-attribute-extensions]
See ‘/data/gannet/ripley/R/packages/tests-clang/image.dlib.Rcheck/00install.out’ for details.