On Tue, Jun 13, 2023 at 10:39:46AM +0200, Marcin Jaczewski wrote:
>wt., 13 cze 2023 o 10:34 Federico Kircheis via Std-Proposals
><std-proposals_at_[hidden]> napisał(a):
>>
>>
>> Hello to everyone, please consider following snippet
>>
>> ----
>> #include <vector>
>> #include <memory>
>>
>> struct s{
>> std::vector<std::unique_ptr<int>> d;
>> };
>>
>> static_assert(!std::is_copy_assignable<std::unique_ptr<int>>::value);
>> static_assert(std::is_copy_assignable<std::vector<std::unique_ptr<int>>>::value);
>> static_assert(std::is_copy_assignable<s>::value);
>>
>> void bar(){
>> s s1;
>> // fails, as expected, but inconsistent with is_copy_assignable
>> auto s2 = s1;
>> }
>> ----
>>
>> Doesn't the second static_assert bother anyone?
>>
>> Vector is always is_copy_assignable, but in fact it is a compile-time
>> error (auto s2 = s1) does not compile.
>>
>> Depending on the compiler, compiler errors are also verbose
>> (https://godbolt.org/z/3E49c4oT7) and not necessarily easy to understand
>> (immagine if s is part of a class hierarchy or a member of another
>> class...)
>>q
>> An improvement for the end-user is to delete the copy-constructor of s
>> by hand, it does not change what programs are valid, but the error
>> message is much more easy to understand.
>>
>> But it does not fix the underlying issue, and the fact that one cannot
>> simply follow the "rule of 0" and let the compiler use the defaulted
>> copy and move constructors without writing them down.
>>
>> Does anyone know if there is already a paper trying to fix this issue,
>> or if there was one and was rejected?
>> I assume the main difficulty is specifying something that also works
>> with forward-declared types.
>>
>> Thanks
>>
>> Federico
>>
>
>You can't have cake and eat it too, "fix" would need to ban
>use of forward declared types in vector:
>```
>struct Foo
>{
> std::vector<Foo> f;
>};
>```
>Now this works fine, after it would not.

Wouldn't it be possible to have vector play well with is_copy_assignable
(ie delete copy-constructor if contained type is not copyable) if the
type is not forward-declared?

Granted, is_copy_assignable could still be wrong with forward-declared
types, but it would at least work correctly for more use-cases.