[Bug c++/96090] New: Inconsistent querying of differring exception specifications of explicitly defaulted functions

[Bug c++/96090] New: Inconsistent querying of differring exception specifications of explicitly defaulted functions

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

            Bug ID: 96090
           Summary: Inconsistent querying of differring exception
                    specifications of explicitly defaulted functions
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/SiGMyX.
```C++
#include <type_traits>
using namespace std;
struct yesthrow_t
{
    yesthrow_t()                              noexcept(false) = default;
    yesthrow_t(const yesthrow_t&)             noexcept(false) = default;
    yesthrow_t(      yesthrow_t&&)            noexcept(false) = default;
    yesthrow_t& operator=(const yesthrow_t&)  noexcept(false) = default;
    yesthrow_t& operator=(      yesthrow_t&&) noexcept(false) = default;
};
static_assert(!is_nothrow_default_constructible_v<yesthrow_t>);
static_assert(!is_nothrow_copy_constructible_v<   yesthrow_t>);
static_assert(!is_nothrow_copy_assignable_v<      yesthrow_t>);
static_assert(!is_nothrow_move_constructible_v<   yesthrow_t>);
static_assert(!is_nothrow_move_assignable_v<      yesthrow_t>);
```
https://gcc.gnu.org/ml/gcc-patches/2019-09/msg00311.html calls out a single
case, but not this inconsistency.

[Bug c++/96090] Inconsistent querying of differring exception specifications of explicitly defaulted functions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-07-07

[Bug c++/96090] noexcept operator of potentially-throwing defaulted function gives the wrong result

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

Johel Ernesto Guerrero Peña <johelegp at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.3.0, 11.2.0, 12.0
           Keywords|                            |rejects-valid
            Summary|Inconsistent querying of    |noexcept operator of
                   |differring exception        |potentially-throwing
                   |specifications of           |defaulted function gives
                   |explicitly defaulted        |the wrong result
                   |functions                   |

--- Comment #1 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Now they all give the wrong answer: https://godbolt.org/z/hroWqTExz.

[Bug c++/96090] noexcept operator of potentially-throwing defaulted function gives the wrong result

[nathanieloshead at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

Nathaniel Shead <nathanieloshead at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nathanieloshead at gmail dot com

--- Comment #2 from Nathaniel Shead <nathanieloshead at gmail dot com> ---
I'm working on a patch for this. But worth noting...

> static_assert(!is_nothrow_default_constructible_v<yesthrow_t>);
I'm not sure this is correct. By https://eel.is/c++draft/meta.unary.prop#9 this
is testing noexceptness of value-initialisation. And
https://eel.is/c++draft/dcl.init.general#9.1 says

> if T has either no default constructor ([class.default.ctor]) or
>    a default constructor that is user-provided or deleted, then
>    the object is default-initialized;
> otherwise, the object is zero-initialized and the semantic constraints
>    for default-initialization are checked, and if T has a non-trivial
>    default constructor, the object is default-initialized;
Since the default constructor here is not user-provided, and also trivial, the
object should not be default-initialized and thus the default constructor is
not called, and thus this is not potentially-throwing.

[Bug c++/96090] noexcept operator of potentially-throwing defaulted function gives the wrong result

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:4719b6f5ae4d758f193a17bbd5fb6cbacd702a23

commit r14-6395-g4719b6f5ae4d758f193a17bbd5fb6cbacd702a23
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sat Oct 28 16:04:52 2023 +1100

    c++: Fix noexcept checking for trivial operations [PR96090]

    This patch stops eager folding of trivial operations (construction and
    assignment) from occurring when checking for noexceptness. This was
    previously done in PR c++/53025, but only for copy/move construction,
    and the __is_nothrow_xible builtins did not receive the same treatment
    when they were added.

    To handle `is_nothrow_default_constructible`, the patch also ensures
    that when no parameters are passed we do value initialisation instead of
    just building the constructor call: in particular, value-initialisation
    doesn't necessarily actually invoke the constructor for trivial default
    constructors, and so we need to handle this case as well.

    This is contrary to the proposed resolution of CWG2820; for now we just
    ensure it matches the behaviour of the `noexcept` operator and create
    testcases formalising this, and if that issue gets accepted we can
    revisit.

            PR c++/96090
            PR c++/100470

    gcc/cp/ChangeLog:

            * call.cc (build_over_call): Prevent folding of trivial special
            members when checking for noexcept.
            * method.cc (constructible_expr): Perform value-initialisation
            for empty parameter lists.
            (is_nothrow_xible): Treat as noexcept operator.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/noexcept81.C: New test.
            * g++.dg/ext/is_nothrow_constructible7.C: New test.
            * g++.dg/ext/is_nothrow_constructible8.C: New test.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

[Bug c++/96090] noexcept operator of potentially-throwing defaulted function gives the wrong result

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96090

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed, I guess.