[Bug libstdc++/51452] New: has_nothrow_.*constructor bugs

[Bug libstdc++/51452] New: has_nothrow_.*constructor bugs

[dave@boost-consulting.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

             Bug #: 51452
           Summary: has_nothrow_.*constructor bugs
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dave@boost-consulting.com


The traits that detect nothrow constructibility are buggy because they are
influenced by whether the object has a nothrow dtor; destruction is invoked at
the end of evaluation of the full expression in the noexcept( ... ) operator. 
They all use the pattern of constructing a temporary inside noexcept, whereas
they should be using placement new:


  struct X
  {
      X() noexcept;
      ~X();
  };

  static_assert( noexcept( X() ), "fails because of ~X" );
  static_assert( noexcept(new (nullptr) X()), "works" );

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-07 17:16:58 UTC ---
I think this is by design, see the thread beginning with c++std-lib-30698

I've been surprised by that reasoning several times e.g.
http://gcc.gnu.org/ml/gcc-help/2011-11/msg00015.html

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-07 17:20:47 UTC ---
Daniel should resolve this.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-07 17:32:54 UTC ---
By the way, Jon, I think we should audit the library a little more
systematically vs PR50043 (assuming that, unfortunately, isn't done in time for
4.7) and in case add explicit noexcept.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-07 17:38:39 UTC ---
yes, I keep forgetting that noexcept should be implied on dtors now

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[dave@boost-consulting.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #5 from dave@boost-consulting.com 2011-12-07 18:41:12 UTC ---
(In reply to comment #1)
> I think this is by design, see the thread beginning with c++std-lib-30698
> 
> I've been surprised by that reasoning several times e.g.
> http://gcc.gnu.org/ml/gcc-help/2011-11/msg00015.html

I see the thread.  I don't see anything in the thread that supports the idea
that it should behave this way, but maybe I'm missing something.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-07 19:03:30 UTC ---
c++std-lib-30708 has Daniel's explanation of his interpretation, as implemented
in GCC.

FWIW I prefer your interpretation, but will peace Daniel to comment further

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #7 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-12-09 15:57:15 UTC ---
Please note that this issue here is a simply a dup of bug 51295, which is a
compiler defect and *not* a library problem. See [class.dtor] p3:

"A declaration of a destructor that does not have an exception-specification is
implicitly considered to have the same exception-specification as an implicit
declaration (15.4)."

I hadn't looked at the actual example until Dave was describing effects on the
LWG reflector that are obviously in contradiction to the core language.

I repeat: The test case should be well-formed with the existing
is_constructible definition once the compiler defect is fixed.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-09 16:02:11 UTC ---
Corrected testcase:

  struct X
  {
      X() noexcept;
      ~X() noexcept(false);
  };

  static_assert( noexcept( X() ), "fails because of ~X" );
  static_assert( noexcept(new (nullptr) X()), "works" );

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |paolo.carlini at oracle dot
                   |                            |com
         Resolution|                            |DUPLICATE

--- Comment #9 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-09 16:03:29 UTC ---
Ah good, let's resolve as duplicate then. Thanks Daniel.

*** This bug has been marked as a duplicate of bug 51295 ***

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[dave at boostpro dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

Dave Abrahams <dave at boostpro dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dave at boostpro dot com

--- Comment #10 from Dave Abrahams <dave at boostpro dot com> 2011-12-09 18:11:41 UTC ---
(In reply to comment #9)
> Ah good, let's resolve as duplicate then. Thanks Daniel.
> 
> *** This bug has been marked as a duplicate of bug 51295 ***

But wait!  The example I gave was not a test case!  The point was that it shows
a defect in traits that detect nothrow constructibility.  This bug should be
re-opened.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2011-12-09
         Resolution|DUPLICATE                   |
     Ever Confirmed|0                           |1

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-09 18:26:57 UTC ---
Agreed (though it should probably be suspended when there's an LWG issue)

My "corrected" testcase wasn't right, here's another attempt:

#include <type_traits>

struct X
{
    X() noexcept;
    ~X() noexcept(false);
};

static_assert(std::is_nothrow_constructible<X>::value, "fails because of ~X");

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #12 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-09 18:37:37 UTC ---
Thus Daniel was wrong when he said that fixing 51295 automatically renders
correct his current implementation of the trait? I suspect there is a
misunderstanding here: we are in control of both compiler and library, thus, if
I understood Daniel correctly, there is no need to keep this issue open, as if
to force some sort of workaround on the library side. Unless we *really* decide
that we don't want to wait for the front-end to be fixed. Anyway, I'm off,
anything Daniel wants to do for his trait, it's Ok with me.

[Bug libstdc++/51452] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-09 19:32:28 UTC ---
The issue is what the "correct" definition of the trait is, but I think we need
a DR to clarify it

[Bug libstdc++/51452] [DR 2116] has_nothrow_.*constructor bugs

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |SUSPENDED
            Summary|has_nothrow_.*constructor   |[DR 2116]
                   |bugs                        |has_nothrow_.*constructor
                   |                            |bugs

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-12 18:14:43 UTC ---
This is http://cplusplus.github.com/LWG/lwg-active.html#2116 so let's suspend
this.

[Bug libstdc++/51452] [DR 2116] has_nothrow_.*constructor bugs

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |cassio.neri at gmail dot
                   |                            |com

--- Comment #15 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-09-26 23:13:59 UTC ---
*** Bug 54722 has been marked as a duplicate of this bug. ***

[Bug libstdc++/51452] [DR 2116] has_nothrow_.*constructor bugs

[antoshkka at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #16 from Antony Polukhin <antoshkka at gmail dot com> 2013-02-04 07:29:16 UTC ---
*** Bug 56191 has been marked as a duplicate of this bug. ***

[Bug libstdc++/51452] [DR 2116] is_nothrow_.*constructible bugs

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #17 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #14)
> This is http://cplusplus.github.com/LWG/lwg-active.html#2116 so let's
> suspend this.

Updated link: https://wg21.link/lwg2116

[Bug libstdc++/51452] [DR 2116] is_nothrow_.*constructible bugs

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452

--- Comment #18 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Related to https://cplusplus.github.io/CWG/issues/2886.html also.