gcc 13 array out of bounds warning in chrono.h

[prlw1]

Out of bounds seems worth reporting even though it is just a warning...
This is with fmt checkout 75f3b1c
Trivial code:

#include <fmt/chrono.h>
#include <fmt/core.h>

int main()
{
        auto tic = std::chrono::high_resolution_clock::now();
        fmt::print("Hello World!\n");
        auto toc = std::chrono::high_resolution_clock::now();
        std::chrono::duration<double, std::milli>tictoc = toc - tic;
        fmt::print("time taken: {}\n", tictoc);
}

gcc checkout 101380a8394c22a7a2ea70de2060ee93716156e2 complains with

/usr/local/bin/g++  -O2 -g -Werror -std=c++23 -march=native -O3     -Wno-error=non-template-friend -Wall -I/usr/src/local/fmt/include  -c    fmttest.cc
In file included from /usr/src/local/fmt/include/fmt/format.h:49,
                 from /usr/src/local/fmt/include/fmt/chrono.h:21,
                 from fmttest.cc:1:
/usr/src/local/fmt/include/fmt/core.h: In instantiation of 'constexpr fmt::v9::detail::value<Context> fmt::v9::detail::make_value(T&&) [with Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; T = std::chrono::duration<double, std::ratio<1, 1000> >&]':
/usr/src/local/fmt/include/fmt/core.h:1704:29:   required from 'constexpr fmt::v9::detail::value<Context> fmt::v9::detail::make_arg(T&&) [with bool IS_PACKED = true; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; type <anonymous> = fmt::v9::detail::type::custom_type; T = std::chrono::duration<double, std::ratio<1, 1000> >&; typename std::enable_if<IS_PACKED, int>::type <anonymous> = 0]'
/usr/src/local/fmt/include/fmt/core.h:1819:77:   required from 'constexpr fmt::v9::format_arg_store<Context, Args>::format_arg_store(T&& ...) [with T = {std::chrono::duration<double, std::ratio<1, 1000> >&}; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; Args = {std::chrono::duration<double, std::ratio<1, 1000> >}]'
/usr/src/local/fmt/include/fmt/core.h:1836:31:   required from 'constexpr fmt::v9::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<_Args>::type>::type ...> fmt::v9::make_format_args(T&& ...) [with Context = basic_format_context<appender, char>; T = {std::chrono::duration<double, std::ratio<1, 1000> >&}]'
/usr/src/local/fmt/include/fmt/core.h:2898:44:   required from 'void fmt::v9::print(format_string<T ...>, T&& ...) [with T = {std::chrono::duration<double, std::ratio<1, 1000> >&}; format_string<T ...> = basic_format_string<char, std::chrono::duration<double, std::ratio<1, 1000> >&>]'
fmttest.cc:10:12:   required from here
/usr/src/local/fmt/include/fmt/core.h:1667:15: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 1667 |   const auto& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |               ^~~
/usr/src/local/fmt/include/fmt/core.h:1667:46: note: the temporary was destroyed at the end of the full expression 'fmt::v9::detail::arg_mapper<fmt::v9::basic_format_context<fmt::v9::appender, char> >().fmt::v9::detail::arg_mapper<fmt::v9::basic_format_context<fmt::v9::appender, char> >::map<std::chrono::duration<double, std::ratio<1, 1000> >&>((* & val))'
 1667 |   const auto& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
In file included from /usr/local/include/c++/13.0.1/algorithm:60,
                 from /usr/src/local/fmt/include/fmt/chrono.h:11:
In static member function 'static constexpr _Up* std::__copy_move<_IsMove, true, std::random_access_iterator_tag>::__copy_m(_Tp*, _Tp*, _Up*) [with _Tp = unsigned int; _Up = unsigned int; bool _IsMove = false]',
    inlined from 'constexpr _OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = unsigned int*; _OI = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_algobase.h:506:30,
    inlined from 'constexpr _OI std::__copy_move_a1(_II, _II, _OI) [with bool _IsMove = false; _II = unsigned int*; _OI = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_algobase.h:533:42,
    inlined from 'constexpr _OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = unsigned int*; _OI = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_algobase.h:540:31,
    inlined from 'constexpr _OI std::copy(_II, _II, _OI) [with _II = unsigned int*; _OI = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_algobase.h:633:7,
    inlined from 'static _ForwardIterator std::__uninitialized_copy<true>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = unsigned int*; _ForwardIterator = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_uninitialized.h:147:27,
    inlined from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = unsigned int*; _ForwardIterator = unsigned int*]' at /usr/local/include/c++/13.0.1/bits/stl_uninitialized.h:185:15,
    inlined from 'constexpr void fmt::v9::basic_memory_buffer<T, SIZE, Allocator>::grow(size_t) [with T = unsigned int; long unsigned int SIZE = 32; Allocator = std::allocator<unsigned int>]' at /usr/src/local/fmt/include/fmt/format.h:951:28,
    inlined from 'constexpr void fmt::v9::detail::buffer<T>::try_reserve(size_t) [with T = unsigned int]' at /usr/src/local/fmt/include/fmt/core.h:921:39,
    inlined from 'constexpr void fmt::v9::detail::buffer<T>::try_resize(size_t) [with T = unsigned int]' at /usr/src/local/fmt/include/fmt/core.h:912:16,
    inlined from 'constexpr void fmt::v9::basic_memory_buffer<T, SIZE, Allocator>::resize(size_t) [with T = unsigned int; long unsigned int SIZE = 32; Allocator = std::allocator<unsigned int>]' at /usr/src/local/fmt/include/fmt/format.h:1022:63,
    inlined from 'constexpr void fmt::v9::detail::bigint::assign(UInt) [with UInt = long unsigned int; typename std::enable_if<(std::is_same<UInt, long unsigned int>::value || std::is_same<_Tp, __int128 unsigned>::value), int>::type <anonymous> = 0]' at /usr/src/local/fmt/include/fmt/format.h:3035:19,
    inlined from 'constexpr void fmt::v9::detail::bigint::operator=(Int) [with Int = int]' at /usr/src/local/fmt/include/fmt/format.h:3056:11,
    inlined from 'constexpr void fmt::v9::detail::bigint::assign_pow10(int)' at /usr/src/local/fmt/include/fmt/format.h:3136:11,
    inlined from 'constexpr void fmt::v9::detail::bigint::assign_pow10(int)' at /usr/src/local/fmt/include/fmt/format.h:3127:24:
/usr/local/include/c++/13.0.1/bits/stl_algobase.h:437:30: error: 'void* __builtin_memmove(void*, const void*, long unsigned int)' forming offset 4 is out of the bounds [0, 4] [-Werror=array-bounds=]
  437 |             __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
      |             ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

(assuming the problem isn't with platform dependent memmove)

[vitaut]

Same as #3354.

[phprus]

@vitaut
What about it?

/usr/src/local/fmt/include/fmt/core.h:1667:15: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 1667 |   const auto& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |               ^~~
/usr/src/local/fmt/include/fmt/core.h:1667:46: note: the temporary was destroyed at the end of the full expression 'fmt::v9::detail::arg_mapper<fmt::v9::basic_format_context<fmt::v9::appender, char> >().fmt::v9::detail::arg_mapper<fmt::v9::basic_format_context<fmt::v9::appender, char> >::map<std::chrono::duration<double, std::ratio<1, 1000> >&>((* & val))'
 1667 |   const auto& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~

[vitaut]

What about it?

I don't see any temporaries there. The argument is passed through by reference.

[phprus]

@vitaut

I don't see any temporaries there. The argument is passed through by reference.

...through by rvalue reference:

fmt/include/fmt/core.h

Lines 1444 to 1447 in f8c9fab

	template <typename T, FMT_ENABLE_IF(formattable<T>::value)>
	FMT_CONSTEXPR FMT_INLINE auto do_map(T&& val) -> T& {
	return val;
	}

Sample from gcc Porting to guide (https://gcc.gnu.org/gcc-13/porting_to.html):

   decltype(auto) f(int&& x) { return (x); }  // returns int&&; previously returned int&
   int& g(int&& x) { return x; }  // ill-formed; previously well-formed

Maybe this is a similar issue?

[jdrouhard]

This does look like a result of the implementation of P2266 in gcc 13. At the very least, I think this issue probably needs to be reopened because fmt-9.1.0 does not compile on gcc-13 with -std=c++20 -Werror as is.

Looks like in c++23, do_map() will be ill formed, and the dangling-reference warning is correct.

[vitaut]

by rvalue reference

It's not an rvalue reference but a forwarding reference. We could probably just forward it to avoid the warning.

[vitaut]

This issue mostly talks about a different warning so I don't think we should reopen it but if the other warning still reproduces feel free to open another issue.