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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111723] New: #pragma GCC system_header suppresses errors
 from narrowing conversions
Date: Sun, 08 Oct 2023 02:13:00 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.1
X-Bugzilla-Keywords: accepts-invalid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: de34 at live dot cn
X-Bugzilla-Status: UNCONFIRMED
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111723

            Bug ID: 111723
           Summary: #pragma GCC system_header suppresses errors from
                    narrowing conversions
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

In the following program, the conversions are narrowing, but only the one f=
or
nonstd::in_fun_result is rejected.

When -Wsystem-headers is used, then the narrowing conversion for
std::ranges::in_fun_result are correctly diagnosed. But if -pedantic-errors=
 and
-Wsystem-headers are used together, some standard headers are rejected.

Godbolt link: https://godbolt.org/z/fT7b16eoe


```
#include <algorithm>
#include <concepts>
#include <utility>

namespace nonstd {
    template<class I, class F>
    struct in_fun_result {
        [[no_unique_address]] I in;
        [[no_unique_address]] F fun;

        template<class I2, class F2>
            requires std::convertible_to<const I&, I2> &&
std::convertible_to<const F&, F2>
        constexpr operator in_fun_result<I2, F2>() const&
        {
            return {in, fun};
        }

        template<class I2, class F2>
            requires std::convertible_to<I, I2> && std::convertible_to<F, F=
2>
        constexpr operator in_fun_result<I2, F2>() &&
        {
            return {std::move(in), std::move(fun)};
        }
    };
}

int main()
{
    std::ranges::in_fun_result<int, int> r1{};
    std::ranges::in_fun_result<short, short> r2 =3D r1; // should be error,=
 but
not diagnosed by default

    nonstd::in_fun_result<int, int> r3{};
    nonstd::in_fun_result<short, short> r4 =3D r3; // error, rejected with
-pedantic-errors
}
```

It seems to me that #pragma GCC system_header shouldn't suppress errors from
narrowing conversions, because the diagnostics are required by the standard=
.=