https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83085
Bug ID: 83085
Summary: Suspicious type qualifiers ignored on cast result type
[-Werror=ignored-qualifiers]
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: andi at mozilla dot com
Target Milestone: ---
I'm using this debian snapshop,https://tracker.debian.org/pkg/gcc-8, of gcc8 in
order to build mozilla firefox, and I encountered this potential issue when
building with -Werror=ignored-qualifiers.
>> 2:46.84 In file included from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Result.h:17:0,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/js/Result.h:119,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/jspubtd.h:22,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/nsCycleCollectionTraversalCallback.h:10,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/nsCycleCollectionNoteChild.h:13,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/nsCOMPtr.h:33,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/nsAutoPtr.h:10,
>> 2:46.84 from
>> /root/firefox-gcc-last/ipc/chromium/src/chrome/common/ipc_message.h:21,
>> 2:46.84 from
>> /root/firefox-gcc-last/ipc/chromium/src/chrome/common/ipc_message_utils.h:22,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/ipc/IPCMessageUtils.h:11,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/ipc/ipdl/_ipdlheaders/mozilla/ipc/IPCStream.h:11,
>> 2:46.84 from
>> /root/firefox-gcc-last/ipc/glue/IPCStreamUtils.h:10,
>> 2:46.84 from
>> /root/firefox-gcc-last/ipc/glue/IPCStreamUtils.cpp:7,
>> 2:46.84 from
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/ipc/glue/Unified_cpp_ipc_glue1.cpp:2:
>> 2:46.84
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Variant.h:
>> In instantiation of 'typename mozilla::detail::Nth<N, Ts ...>::Type
>> mozilla::Variant<Ts>::extract() [with long unsigned int N = 0; Ts = {const
>> int, const char*, void (*)(nsITimer*, bool, void*, char*, long unsigned
>> int)}; typename mozilla::detail::Nth<N, Ts ...>::Type = const int]':
>> 2:46.84
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Variant.h:219:7:
>> required from 'static void mozilla::detail::VariantImplementation<Tag, N,
>> T, Ts ...>::moveConstruct(void*, Variant&&) [with Variant =
>> mozilla::Variant<const int, const char*, void (*)(nsITimer*, bool, void*,
>> char*, long unsigned int)>; Tag = unsigned char; long unsigned int N = 0; T
>> = const int; Ts = {const char*, void (*)(nsITimer*, bool, void*, char*, long
>> unsigned int)}]'
>> 2:46.84
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Variant.h:588:24:
>> required from 'mozilla::Variant<Ts>::Variant(mozilla::Variant<Ts>&&)
>> [with Ts = {const int, const char*, void (*)(nsITimer*, bool, void*, char*,
>> long unsigned int)}]'
>> 2:46.84 /usr/include/c++/8/bits/move.h:193:11: required from 'typename
>> std::enable_if<std::__and_<std::__not_<std::__is_tuple_like<_Tp> >,
>> std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp>
>> >::value>::type std::swap(_Tp&, _Tp&) [with _Tp = mozilla::Variant<const
>> int, const char*, void (*)(nsITimer*, bool, void*, char*, long unsigned
>> int)>; typename
>> std::enable_if<std::__and_<std::__not_<std::__is_tuple_like<_Tp> >,
>> std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp>
>> >::value>::type = void]'
>> 2:46.84 /root/firefox-gcc-last/xpcom/threads/nsTimerImpl.h:98:35: required
>> from here
>> 2:46.84
>> /root/firefox-gcc-last/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Variant.h:714:44:
>> error: type qualifiers ignored on cast result type
>> [-Werror=ignored-qualifiers]
>> 2:46.84 return typename detail::Nth<N, Ts...>::Type(Move(as<N>()));
>> 2:46.84
Looking at the code the issue is reported here:
https://dxr.mozilla.org/mozilla-central/source/mfbt/Variant.h?q=return+typename+detail%3A%3ANth%3CN%2C+Ts...%3E%3A%3AType%28Move%28as%3CN%3E%28%29%29%29%3B&redirect_type=single#709
That gets called from here:
https://dxr.mozilla.org/mozilla-central/source/xpcom/threads/nsTimerImpl.h?q=nsTimerImpl.h&redirect_type=direct#98
I personally believe that in this context the warning just adds noise since
nothing dangerous nor standard compliant happens.
