[Bug c++/64791] New: Generic lambda fails to implicitly capture `const` variable

[Bug c++/64791] New: Generic lambda fails to implicitly capture `const` variable

[tom at kera dot name]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

            Bug ID: 64791
           Summary: Generic lambda fails to implicitly capture `const`
                    variable
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tom at kera dot name

>From http://stackoverflow.com/q/28141403/560648

Reproduction:

/////////////////////////////
#include <iostream>
#include <functional>

int main()
{
    const int a = 2;
    std::function<void(int)> f = [&](auto b) { std::cout << a << ", " << b <<
std::endl; };
    f(3);
}
/////////////////////////////


Taking any of the following steps allows the program to build and run with the
expected output "2, 3":

- remove `const` from declaration of a
- name `a` in the capture-list instead of relying on implicit capture
- change declaration of `f` from `std::function<void(int)>` to `auto`
- make the lambda non-generic by changing `auto b` to `int b`
- use Clang (e.g. v3.5.0)

Suspect detection of odr-use is breaking, or this could be related to bug
61814, or something else entirely?

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[tom at kera dot name]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #1 from Tomalak Geret'kal <tom at kera dot name> ---
Build error:

/////////////////////////////
main.cpp: In instantiation of 'main()::<lambda(auto:1)> [with auto:1 = int]':
/usr/local/include/c++/4.9.2/functional:2149:71:   required by substitution of
'template<class _Res, class ... _ArgTypes> template<class _Functor> using
_Invoke = decltype
(std::__callable_functor(declval<_Functor&>())((declval<_ArgTypes>)()...))
[with _Functor = main()::<lambda(auto:1)> _Res = void; _ArgTypes = {int}]'
/usr/local/include/c++/4.9.2/functional:2225:9:   required by substitution of
'template<class _Functor, class> std::function<_Res(_ArgTypes
...)>::function(_Functor) [with _Functor = main()::<lambda(auto:1)>
<template-parameter-1-2> = <missing>]'
main.cpp:7:90:   required from here
main.cpp:7:58: error: 'a' was not declared in this scope
     std::function<void(int)> f = [&](auto b) { std::cout << a << ", " << b <<
std::endl; };
                                                          ^
/////////////////////////////

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[tom at kera dot name]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #2 from Tomalak Geret'kal <tom at kera dot name> ---
Actually, I'm no longer sure that `a` *is* odr-used...

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-01-27
                 CC|                            |paolo.carlini at oracle dot com,
                   |                            |ville.voutilainen at gmail dot com
      Known to work|                            |5.0
     Ever confirmed|0                           |1
      Known to fail|                            |4.8.2, 4.9.1

--- Comment #3 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Trunk accepts the code and prints 2, 3 like clang does. So unless there's a
need for an additional testcase or a backport, this looks like a candidate for
closing.

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #4 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Oh, wait a minute, with trunk, I see an incorrect warning:

prog.cc: In function 'int main()':
prog.cc:7:15: warning: variable 'a' set but not used
[-Wunused-but-set-variable]
    const int a = 2;
              ^

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #6 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
I ran it with this thing:
http://melpon.org/wandbox/permlink/gAsh89NaSSFspYjq

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #7 from Paolo Carlini <paolo.carlini at oracle dot com> ---
I know that web page, but today, 20150131, I can't reproduce the issue with the
current tree on my machine.

[Bug c++/64791] Generic lambda fails to implicitly capture `const` variable

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64791

--- Comment #9 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
I can't see any warnings with my trunk build from today, either.