[Bug libstdc++/57049] New: std::swap does self move assignment, which is illegal

[Bug libstdc++/57049] New: std::swap does self move assignment, which is illegal

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57049

             Bug #: 57049
           Summary: std::swap does self move assignment, which is illegal
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tudorb@fb.com


Created attachment 29922
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29922
sample program that illustrates incorrect behavior

The default implementation of std::swap does self-move-assignment when calling
swap(a, a), which is illegal according to 17.6.4.9.

[Bug libstdc++/57049] std::swap does self move assignment, which is illegal

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57049

--- Comment #1 from Tudor Bosman <tudorb at fb dot com> 2013-04-23 15:54:18 UTC ---
Actually, I'll take this back.  I don't believe this is a bug.

17.6.4.9 constraints arguments passed to STL functions.  So if there is a
library function that takes a rvalue argument (such as
string::operator=(string&& other)), then the library is free to assume that the
argument doesn't alias (so &other != this).  So the assertion in basic_fbstring
is correct.

The default implementation of swap() does self-move assignment.  That would be
illegal if the types were MoveAssignable STL types (because then they'd call
T::operator=(T&&) which would be illegal according to 17.6.4.9) BUT ALL STL
TYPES HAVE SPECIALIZED IMPLEMENTATIONS OF SWAP which don't do self move
assignment.

So the default swap() will do self-move-assignment on user types, but there's
no language in the standard that bans self-move-assignment there.

[Bug libstdc++/57049] std::swap does self move assignment, which is illegal

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57049

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

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

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2013-04-25 09:55:47 UTC ---
Here are two different things to consider:

1) The standard library specification of basic_string's move-assignment
operator currently requires that it works for self-assignment:

<quote>
21 Effects: If *this and str are not the same object, modifies *this as shown
in Table 71. [...]

22 If *this and str are the same object, the member has no effect.
</quote>

This might or might not change, depending on the outcome of LWG issue

http://cplusplus.github.io/LWG/lwg-active.html#2063

2) The generic swap template from <utility> imposes requirements on the
argument types and describes what it does (which can only rely on the
requirements). So if you call swap with the same argument value and if for
these argument values the move-assignment is undefined behaviour, you get what
you are asking for. There is no single evidence that swap would be even allowed
to prevent swapping for identical arguments, because the semantics are
specified clearly to exchange the two values.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW065

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-04-25 10:01:40 UTC ---
Good catch, thanks, Kenny

[Bug libstdc++/57049] std::swap does self move assignment, which is illegal

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57049

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Closing then.