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From: "sim.f.nilsson at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113158] New: Erroneous "looser exception specification"
 error for class template
Date: Wed, 27 Dec 2023 17:38:46 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: sim.f.nilsson at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113158

            Bug ID: 113158
           Summary: Erroneous "looser exception specification" error for
                    class template
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sim.f.nilsson at gmail dot com
  Target Milestone: ---

The following code is not accepted by GCC 13.2.0 (among other versions):

#include <type_traits>

struct base {
    virtual int f() const =3D 0;
};

template<typename T>
struct derived : base {
    int f() const noexcept(std::is_nothrow_copy_constructible_v<T>) override
    {
        return sizeof(T);
    }
};

int g()
{
    return derived<double>{}.f();
}

I've tested versions 9 through 13 and trunk. Version 9.5 seems to be the la=
st
version that accepts the code. The code is accepted by both Clang and MSVC.=
 To
the best of my knowledge Clang and MSVC are correct to accept the code as
base::f is a "potentially throwing function" without the noexcept specifier.

What flags are used when compiling the code does not seem to influence the
behaviour, but for reference I've mostly used '-Wall -Wextra -O1 -std=3Dc++=
17' in
comparisons.

The following compiler-explorer link also demonstrates the error:
https://compiler-explorer.com/z/1c48Yr76P

Adding 'noexcept(false)' in the base-class declaration of f, or either
'noexcept' or 'noexcept(false)' to the derived definition, makes GCC accept=
 it.=