clang's -Wbitfield-enum-conversion warning reports when an enum value is assigned to a bit field that is not wide enough to store all the enum values: https://clang.llvm.org/docs/DiagnosticsReference.html#wbitfield-enum-conversion This warning found some real layout style bugs where a 4-bit enum was implicitly truncated to a 3-bit bit field: layout/tables/nsTableFrame.cpp:5318:14 [-Wbitfield-enum-conversion] bit-field 'ownerElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5358:16 [-Wbitfield-enum-conversion] bit-field 'ownerElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5374:18 [-Wbitfield-enum-conversion] bit-field 'subElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5385:18 [-Wbitfield-enum-conversion] bit-field 'subElem' is not wide enough to store all enumerators of 'BCBorderOwner' I fixed the bit fields (bug 1485179), which unfortunately revealed some underlying layout bugs (bug 1508921) that depended on the buggy behavior.

https://clang.llvm.org/docs/DiagnosticsReference.html#wbitfield-enum-conversion This clang warning caught a real layout bug related to implicitly truncated enums in the table border style code (bug 1485179): layout/tables/nsTableFrame.cpp:5318:14 [-Wbitfield-enum-conversion] bit-field 'ownerElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5358:16 [-Wbitfield-enum-conversion] bit-field 'ownerElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5374:18 [-Wbitfield-enum-conversion] bit-field 'subElem' is not wide enough to store all enumerators of 'BCBorderOwner' layout/tables/nsTableFrame.cpp:5385:18 [-Wbitfield-enum-conversion] bit-field 'subElem' is not wide enough to store all enumerators of 'BCBorderOwner' Also update the comment linking to clang and gcc's warning documentation.

Pushed by cpeterson@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/6feb94524b7e Enable clang's -Wbitfield-enum-conversion warnings. r=glandium