Forward declaration and std::future

[Jonathan Wakely]

On Sun, 12 Sep 2021, 19:30 Evgen Bodunov, <evgen@globus.software> wrote:

> Hello Jonathan,
>
> Please, could you refer exact part and version of the standard?
>


Please reply to the mailing list, not just to me.

Every version of the standard has the same requirement, see
http://eel.is/c++draft/requirements#res.on.functions-2.5

std::promise requires its result type to be destructible. We are allowed to
assert that by using is_destructible because we can assume the type is
complete, because [res.on.functions] says it must be.

In a correct program, the assertion will not fail.





> On 12.09.2021, at 20:19, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
>
>
> On Sun, 12 Sep 2021, 10:36 Evgen Bodunov, <evgen@globus.software> wrote:
>
>> Hello everyone,
>>
>> During development of valhalla routing project I stumbled upon
>> compilation error on gcc + libstdc++ 11 and newer when std::future
>> constructed from forward declared class. Live example is there:
>> https://godbolt.org/z/rEx3Eonsz It builds on gcc prior to v11. and on
>> any clang with libc++.
>>
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits: In
>> instantiation of 'struct std::is_destructible<test_data>':
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:
>>  required from 'class std::promise<test_data>'
>> <source>:8:29:   required from here
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52:
>> error: static assertion failed: template argument must be a complete class
>> or an unbounded array
>>   849 |
>>  static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>>       |
>>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52:
>> note: 'std::__is_complete_or_unbounded<std::__type_identity<test_data>
>> >((std::__type_identity<test_data>{}, std::__type_identity<test_data>()))'
>> evaluates to false
>> In file included from <source>:1:
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future: In
>> instantiation of 'class std::promise<test_data>':
>> <source>:8:29:   required from here
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:
>> error: static assertion failed: result type must be destructible
>>  1065 |       static_assert(is_destructible<_Res>{},
>>       |                     ^~~~~~~~~~~~~~~~~~~~~~~
>> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:
>> note: 'std::is_destructible<test_data>{}' evaluates to false
>> Compiler returned: 1
>>
>> Is that static assert is really required? May be then it should correctly
>> process forward declared types?
>>
>
> The standard says your code has undefined behaviour. The assertion is
> pointing out your mistake.
>
>
>
>