[cfe-users] Code which compiles with g++ but not with clang++

[connor horman via cfe-users]

On Thu, Nov 5, 2020 at 11:06 connor horman <chorman64 at gmail.com> wrote:

> Looking at https://eel.is/c++draft/optional.optional and optional.assign,
> it shouldn't be ill-formed to have a non copy-constructible type, the copy
> constructor of optional should be defined as deleted.
> [optional.optional.general] clause 3 "T shall be a type other than cv
> in_place_t or cv nullopt_t that meets the Cpp17Destructible requirements"
> [optional.assign] clause 7 "This operator is defined as deleted unless
> is_copy_constructible_v<T> is true and is_copy_assignable_v<T> is true"
>
> I presume it's the same or similar in both C++17 and C++20 (published)
> (with Cpp17Destructible being just Destructible in C++17).
>
>
> On Thu, Nov 5, 2020 at 10:07 Pavel Černohorský via cfe-users <
> cfe-users at lists.llvm.org> wrote:
>
>> On 05. 11. 20 15:47, Marshall Clow via cfe-users wrote:
>>
>> I tried this code on compiler explorer.
>> It compiles w/o error with clang + libc++
>> It fails to compile with clang and libstdc++
>>
>> The error message given is:
>>
>> In file included from <source>:1:
>> /opt/compiler-explorer/gcc-10.2.0/lib/gcc/x86_64-linux-gnu/10.2.0/../../../../include/c++/10.2.0/optional:158:7:
>> error: the parameter for this explicitly-defaulted copy assignment operator
>> is const, but a member or base requires it to be non-const
>>      operator=(const _Optional_payload_base&) = default;
>>      ^
>> /opt/compiler-explorer/gcc-10.2.0/lib/gcc/x86_64-linux-gnu/10.2.0/../../../../include/c++/10.2.0/optional:357:7:
>> note: in instantiation of template class 'std::_Optional_payload_base<S>'
>> requested here
>>    : _Optional_payload_base<_Tp>
>>      ^
>> /opt/compiler-explorer/gcc-10.2.0/lib/gcc/x86_64-linux-gnu/10.2.0/../../../../include/c++/10.2.0/optional:631:30:
>> note: in instantiation of template class 'std::_Optional_payload<S, true,
>> false, false>' requested here
>>      _Optional_payload<_Tp> _M_payload;
>>                             ^
>> /opt/compiler-explorer/gcc-10.2.0/lib/gcc/x86_64-linux-gnu/10.2.0/../../../../include/c++/10.2.0/optional:660:15:
>> note: in instantiation of template class 'std::_Optional_base<S, true,
>> true>' requested here
>>    : private _Optional_base<_Tp>,
>>              ^
>> <source>:10:20: note: in instantiation of template class
>> 'std::optional<S>' requested here
>>  std::optional<S> a;
>>                   ^
>> 1 error generated.
>>
>> I got exactly the same error, but the question is why. Shouldn't both g++
>> and clang++ behave the same way even on the libstdc++ (which is the default
>> library clang uses on a lot of distros)? Is this gcc being too permissive
>> to some bug in libstdc++ or is it a clang bug? Should I file a bug report
>> to clang?
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>
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