But its now used in ranges::view_interface, requires closure checked in
place of usage, not in place of declaration

вт, 13 июн. 2023 г. в 12:40, Marcin Jaczewski via Std-Proposals <
std-proposals_at_[hidden]>:

> 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...)
> >
> > 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.
>
> > --
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