[Bug c++/63217] New: template conversion operator returning const reference not used for conversion in some cases

[Bug c++/63217] New: template conversion operator returning const reference not used for conversion in some cases

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

            Bug ID: 63217
           Summary: template conversion operator returning const reference
                    not used for conversion in some cases
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com

Minimized repro:

struct foo {
    template<class T>
    operator const T & () { static T t {}; return t;}
};

int main() {
    int t((foo()));
}

g++ complains:

prog.cc: In function 'int main()':
prog.cc:7:18: error: cannot convert 'foo' to 'int' in initialization
     int t((foo()));
                  ^

This behavior is consistently reproducible in all versions of g++ I tested.
Both Clang and MSVC 2013 compiles this code without errors.

The template `operator const T&` should have been instantiated with `T = int`
and used for the conversion. Using a non-template `operator const int &`
instead of a template makes the code compile.

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
The same problem exist in the current trunk (Tested for 5.0.0 20141002
(experimental))
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Subject: [Bug lto/63409] [5 Regression] FAIL: g++.dg/lto/pr63270 cp_lto_pr63270_0.o-cp_lto_pr63270_1.o link, -flto -O2 -Wno-odr -fno-linker-plugin
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63409

--- Comment #3 from Martin Liška <mliska at suse dot cz> ---
Created attachment 33643
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33643&action=edit
Fix patch
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63409

--- Comment #4 from Martin Liška <mliska at suse dot cz> ---
Can you please verify for me that the following patch fixes the problem for
your arch?

Thanks,
Martin
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc435

David Edelsohn <dje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-10-03
     Ever confirmed|0                           |1

--- Comment #3 from David Edelsohn <dje at gcc dot gnu.org> ---
I have not seen any problems, although the coverage is limited.

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-12-13
                 CC|                            |ville.voutilainen at gmail dot com
     Ever confirmed|0                           |1
      Known to fail|                            |4.8.2, 4.9.1, 5.0

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=85250
   Last reconfirmed|2017-01-08 00:00:00         |2021-8-9

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a C++98 version which shows this has always been rejected (well at
least since 4.1.2):
struct foo {
    template<class T>
    operator const T & () { static T t =0; return t;}
};
int t = foo();

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vz at quantitativesystems dot com

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 79035 has been marked as a duplicate of this bug. ***

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
ICC also rejects this for the same reason as GCC.
MSVC and clang both accept it though.

Could there be a defect report for this?

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |94404
              Alias|                            |CWG976

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#976


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94404
[Bug 94404] [meta-bug] C++ core issues

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Other related ones:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#493
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#322
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#913

[Bug c++/63217] template conversion operator returning const reference not used for conversion in some cases

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 61663.

*** This bug has been marked as a duplicate of bug 61663 ***