[Bug c++/62172] New: Operand to a throw expression is moved when it should be copied

[Bug c++/62172] New: Operand to a throw expression is moved when it should be copied

[tsimpson at ubuntu dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62172

            Bug ID: 62172
           Summary: Operand to a throw expression is moved when it should
                    be copied
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tsimpson at ubuntu dot com

Created attachment 33352
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33352&action=edit
test case

With a throw expression whos operand is an lvalue, which is declared outside
the try block, will move from the expression instead of copy.

The relavent part of the standard is 15.1 [except.throw].
The C++11 standard *could* be interpreded to allow the move when reading the
last sentence of 15.1/3
"...Except for these restrictions and the restrictions on type matching
mentioned in 15.3, the operand of throw is treated exactly as a function
argument in a call (5.2.2) or the operand of a return statement."
Specifically, if the operand of the throw expression was treated exactly the
same as an operand to a return statement then it would become an xvalue and the
move would be allowed.

The wording of that clause changed in C++14, so this is no longer the case. And
I don't think it was the intent to allow that in C++11 either.

Example output with g++ http://rextester.com/USFXJ5678
Exmaple output with clang++ http://rextester.com/EARQ18237

[Bug c++/62172] Operand to a throw expression is moved when it should be copied

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62172

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-08-18
                 CC|                            |ville.voutilainen at gmail dot com
     Ever confirmed|0                           |1

--- Comment #1 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Reduced:

#include <cassert>

int move_count = 0;
struct X {
  X() = default;
  X(const X&) = default;
  X(X&&) {++move_count;}
};

int main()
{
  X x;
  try {throw x;}
  catch (X&) {}
  assert(move_count==0);
}

Asserts on gcc, doesn't assert on clang.

[Bug c++/62172] Operand to a throw expression is moved when it should be copied

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62172

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think this is a DUP of an existing report

[Bug c++/62172] Operand to a throw expression is moved when it should be copied

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62172

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

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

--- Comment #3 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Dup, the testcases are almost identical.

*** This bug has been marked as a duplicate of bug 57533 ***