[Bug c++/49372] New: Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[Bug c++/49372] New: Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

           Summary: Temporaries evaluated for arguments of a default
                    constructors of array elements not destructed properly
                    (?)
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: schaub.johannes@googlemail.com


This testcase outputs a surprising result:

struct A {
  A() { std::cout << "C" << std::endl; }
  ~A() { std::cout << "D" << std::endl; }
};

struct B {
  B(A const& a = A()) { }
};

typedef B array[2];

int main() {
  array{};
}

GCC prints "CCDD". I would have expected "CDCD", as it seems that is the intent
of the wording in the spec (FDIS). The FDIS however seems to be contradicting
here, but it appears to me that GCC should better print "CDCD", so as to follow
the intent. 

Originated from
http://stackoverflow.com/questions/6315670/when-an-array-is-created-by-a-subexpression-what-happens-with-the-temporaries-th
.

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

--- Comment #1 from Johannes Schaub <schaub.johannes at googlemail dot com> 2011-06-11 13:46:46 UTC ---
To elaborate on it, I have the following weird behavior:

- GCC4.6 outputs nothing for the program (on my linux machine). That seems
definitely wrong in any case.
- GCC4.7 "4.7.0 20110517 (experimental)", using a mingw nightly build,
generates an executable that crashes when running:

[js@HOST2 cpp]$ ~/w64/bin/i686-w64-mingw32-g++ -std=c++0x -O1 -o a.exe
main1.cpp
[js@HOST2 cpp]$ wine ./a.exe
Cwine: Unhandled page fault on read access to 0x00000000 at address (nil)
(thread 0025), starting debugger...

- The same GCC4.7 when using -O2 does not crash and print "CCDD":

[js@HOST2 cpp]$ ~/w64/bin/i686-w64-mingw32-g++ -std=c++0x -O2 -o a.exe
main1.cpp
[js@HOST2 cpp]$ wine ./a.exe
fixme:ntoskrnl:KeInitializeSpinLock stub: 0x5477a4
C
C
D
D

I don't know precisely whether this is a problem with wine or a temporary
problem with that nightly build of GCC I was using. My apologies if GCC trunk
works differently!

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[ktietz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

Kai Tietz <ktietz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |ktietz at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #2 from Kai Tietz <ktietz at gcc dot gnu.org> 2012-12-10 11:06:49 UTC ---
This testcase seems to produce for trunk, 4.7, and 4.6 the output CCDD, which
is proper as constructor of B is called by the array-constructor.
Btw you will need option -std=c++11 on trunk to get this test compile without
issues.

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-12-10 13:14:56 UTC ---
Kai, I don't think anyone disputes that B's constructor is called, the question
is why 12.2/4 doesn't apply.

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[ktietz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

Kai Tietz <ktietz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |NEW
   Last reconfirmed|                            |2012-12-10
         Resolution|INVALID                     |
     Ever Confirmed|0                           |1

--- Comment #4 from Kai Tietz <ktietz at gcc dot gnu.org> 2012-12-10 13:42:30 UTC ---
(In reply to comment #3)
> Kai, I don't think anyone disputes that B's constructor is called, the question
> is why 12.2/4 doesn't apply.

Well, we have here 5.2.2 which says "A function call is a postfix expression
followed by parentheses containing a possibly empty, comma-separated list of
expressions which constitute the arguments to the function."

But we don't have here an explicit function-call.  And an implicit call is not
an expression, hence, not a part of any full expression, and so we don't have a
sequence-point.  Not sure if specification intended it differently, but by its
current wording I would assume that current implementation is right.

But indeed 12.4 seems to be violated here.  "There are two contexts in which
temporaries are destroyed at a different point than the end of the
fullexpression.
The first context is when a default constructor is called to initialize an
element of an array. If the constructor has one or more default arguments, the
destruction of every temporary created in a default argument expression is
sequenced before the construction of the next array element, if any."

So there is an antagonism in spec.  I will reopen it.  sorry, didn't read about
12.4

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

--- Comment #5 from Johannes Schaub <schaub.johannes at googlemail dot com> 2012-12-10 16:42:59 UTC ---
(In reply to comment #4)
> (In reply to comment #3)
> > Kai, I don't think anyone disputes that B's constructor is called, the question
> > is why 12.2/4 doesn't apply.
> 
> Well, we have here 5.2.2 which says "A function call is a postfix expression
> followed by parentheses containing a possibly empty, comma-separated list of
> expressions which constitute the arguments to the function."
> 
> But we don't have here an explicit function-call.  And an implicit call is not
> an expression, hence, not a part of any full expression, and so we don't have a
> sequence-point.  Not sure if specification intended it differently, but by its
> current wording I would assume that current implementation is right.
> 

See 1.9p10. The implicit function call is considered to be an expression.

[Bug c++/49372] Temporaries evaluated for arguments of a default constructors of array elements not destructed properly (?)

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49372

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=92385
   Target Milestone|---                         |12.0

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
It seems this is fixed for GCC 12+ by r12-6326-ge948436eab818c.