[Bug c++/62227] New: Templated move not elided

[Bug c++/62227] New: Templated move not elided

[roman.perepelitsa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62227

            Bug ID: 62227
           Summary: Templated move not elided
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roman.perepelitsa at gmail dot com

#include <stdio.h>

struct S {
  S(int) {}
  S(const S&) = default;   // (1)
  template <class = void>  // (2)
  S(S&& other) {
    puts("move");
  }
};

int main() {
  S s = 42;
  (void)s;
}

This program unexpectedly prints "move". The expected output is empty (move is
elided).

The program prints nothing (as expected) if any of the following is done:

  - Line (1) is removed.
  - Line (2) is removed.
  - The program is compiled with clang.

[Bug c++/62227] Templated move not elided

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62227

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A template cannot be a move constructor and so it is not legal to elide it.

Removing line (1) means there is a move constructor implicitly declared, which
will be used and can be elided.

Removing line (2) means there is a move constructor, which will be used and
elided.

[Bug c++/62227] Templated move not elided

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62227

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C++11 says "When certain criteria are met, an implementation is allowed to omit
the copy/move construction of a class object, even if the copy/move
constructor..." which only applies to copy constructors or move constructors.


But the current draft says "even if the constructor selected for the copy/move
operation"

So it does seem that elision is now allowed when the relevant constructor is a
template.

[Bug c++/62227] Templated move not elided

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62227

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#535 changed that
wording

[Bug c++/62227] [DR535] Templated move not elided

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62227

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
C++17 and C++20 modes no longer print move since GCC 7.
Most likely due to the patches to implement p0135.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0135r1.html