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Suppress a false positive in gcc
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@vitaut
vitaut committed May 6, 2023
1 parent 192df93 commit f61f15c
Showing 1 changed file with 6 additions and 6 deletions.
12 changes: 6 additions & 6 deletions include/fmt/core.h
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Original file line number Diff line number Diff line change
Expand Up @@ -785,7 +785,7 @@ FMT_MODULE_EXPORT template <typename Context> class basic_format_args;
FMT_MODULE_EXPORT template <typename Context> class dynamic_format_arg_store;

// A formatter for objects of type T.
FMT_MODULE_EXPORT
FMT_MODULE_EXPORT
template <typename T, typename Char = char, typename Enable = void>
struct formatter {
// A deleted default constructor indicates a disabled formatter.
Expand Down Expand Up @@ -1671,22 +1671,22 @@ constexpr auto encode_types() -> unsigned long long {

template <typename Context, typename T>
FMT_CONSTEXPR FMT_INLINE auto make_value(T&& val) -> value<Context> {
const auto& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
using arg_type = remove_cvref_t<decltype(arg)>;

constexpr bool formattable_char =
!std::is_same<decltype(arg), const unformattable_char&>::value;
!std::is_same<arg_type, unformattable_char>::value;
static_assert(formattable_char, "Mixing character types is disallowed.");

// Formatting of arbitrary pointers is disallowed. If you want to format a
// pointer cast it to `void*` or `const void*`. In particular, this forbids
// formatting of `[const] volatile char*` printed as bool by iostreams.
constexpr bool formattable_pointer =
!std::is_same<decltype(arg), const unformattable_pointer&>::value;
!std::is_same<arg_type, unformattable_pointer>::value;
static_assert(formattable_pointer,
"Formatting of non-void pointers is disallowed.");

constexpr bool formattable =
!std::is_same<decltype(arg), const unformattable&>::value;
constexpr bool formattable = !std::is_same<arg_type, unformattable>::value;
static_assert(
formattable,
"Cannot format an argument. To make type T formattable provide a "
Expand Down

5 comments on commit f61f15c

@gendalph
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@gendalph gendalph commented on f61f15c May 10, 2023 •
edited
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this fix doesn't change anything, the warning remains

@gendalph
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[1/1] /usr/bin/c++ -DGTEST_HAS_STD_WSTRING=1 -D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING=1 -I~/src/fmt/include -isystem ~/src/fmt/test/gtest/. -isystem ~/src/fmt/test/gtest/googlemock/include -isystem ~/src/fmt/test/gtest/googletest/include -isystem ~/src/fmt/test/gtest/googletest -isystem ~/src/fmt/test/gtest/googlemock -O2 -DNDEBUG -std=gnu++20 -fvisibility=hidden -fvisibility-inlines-hidden -Wno-stringop-overflow -pedantic-errors -Wall -Wextra -pedantic -Wold-style-cast -Wundef -Wredundant-decls -Wwrite-strings -Wpointer-arith -Wcast-qual -Wformat=2 -Wmissing-include-dirs -Wcast-align -Wctor-dtor-privacy -Wdisabled-optimization -Winvalid-pch -Woverloaded-virtual -Wconversion -Wno-ctor-dtor-privacy -Wno-format-nonliteral -Wno-dangling-else -Wno-unused-local-typedefs -Wdouble-promotion -Wtrampolines -Wzero-as-null-pointer-constant -Wuseless-cast -Wvector-operation-performance -Wsized-deallocation -Wshadow -Wshift-overflow=2 -Wnull-dereference -Wduplicated-cond -MD -MT test/CMakeFiles/args-test.dir/args-test.cc.o -MF test/CMakeFiles/args-test.dir/args-test.cc.o.d -o test/CMakeFiles/args-test.dir/args-test.cc.o -c ~/src/fmt/test/args-test.cc
In file included from ~/src/fmt/include/fmt/args.h:15,
                 from ~/src/fmt/test/args-test.cc:8:
~/src/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 = const custom_type&]':
~/src/fmt/include/fmt/core.h:1701:35:   required from 'constexpr fmt::v9::basic_format_arg<Context> fmt::v9::detail::make_arg(T&&) [with Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; T = const custom_type&]'
~/src/fmt/include/fmt/args.h:125:49:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::emplace_arg(const T&) [with T = custom_type; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/include/fmt/args.h:167:18:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::push_back(const T&) [with T = custom_type; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/test/args-test.cc:55:18:   required from here
~/src/fmt/include/fmt/core.h:1674:10: warning: possibly dangling reference to a temporary [-Wdangling-reference]
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |          ^~~
~/src/fmt/include/fmt/core.h:1674:41: 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<const custom_type&>((* & val))'
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
~/src/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 = const to_stringable&]':
~/src/fmt/include/fmt/core.h:1701:35:   required from 'constexpr fmt::v9::basic_format_arg<Context> fmt::v9::detail::make_arg(T&&) [with Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; T = const to_stringable&]'
~/src/fmt/include/fmt/args.h:125:49:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::emplace_arg(const T&) [with T = to_stringable; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/include/fmt/args.h:167:18:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::push_back(const T&) [with T = to_stringable; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/test/args-test.cc:84:18:   required from here
~/src/fmt/include/fmt/core.h:1674:10: warning: possibly dangling reference to a temporary [-Wdangling-reference]
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |          ^~~
~/src/fmt/include/fmt/core.h:1674:41: 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<const to_stringable&>((* & val))'
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
~/src/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 = const copy_throwable&]':
~/src/fmt/include/fmt/core.h:1701:35:   required from 'constexpr fmt::v9::basic_format_arg<Context> fmt::v9::detail::make_arg(T&&) [with Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; T = const copy_throwable&]'
~/src/fmt/include/fmt/args.h:125:49:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::emplace_arg(const T&) [with T = copy_throwable; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/include/fmt/args.h:167:18:   required from 'void fmt::v9::dynamic_format_arg_store<Context>::push_back(const T&) [with T = copy_throwable; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>]'
~/src/fmt/test/args-test.cc:171:20:   required from here
~/src/fmt/include/fmt/core.h:1674:10: warning: possibly dangling reference to a temporary [-Wdangling-reference]
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |          ^~~
~/src/fmt/include/fmt/core.h:1674:41: 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<const copy_throwable&>((* & val))'
 1674 |   auto&& arg = arg_mapper<Context>().map(FMT_FORWARD(val));
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~

@vitaut
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@vitaut vitaut commented on f61f15c May 10, 2023

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this fix doesn't change anything, the warning remains

It fixes the warning reported in #3415 which is trivial to check: https://godbolt.org/z/ajsGKe4nE. If you have another example please provide a godbolt repro.

@rubidium42
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I do not know whether this has the same root cause as gendalph's issue, but it's at least very similar, The important bit in the code is the const for the path; without it, it does not create a warning and with it, it does.

#include <fmt/format.h>
#include <fmt/std.h>

int main() {
    const std::filesystem::path from("");
    fmt::print("{}", from);
    return 0;
}

It can also be found at: https://godbolt.org/z/KhcYo4q8z
I'm not sure whether it reproduces on trunk, but I run into the "possibly dangling reference to a temporary" warning at line 1674 of this diff with version 10.0.0.

@vitaut
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@vitaut vitaut commented on f61f15c May 22, 2023

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@rubidium42, this false positive is also suppressed now (ef55d4f).

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