[cfe-dev] [libc++] Potential bug in debug/array

[Liu Hao via cfe-dev]

在 2018/5/18 0:09, Lucas Czech via cfe-dev 写道:
> Error message:
> 
> |/usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/debug/array:86:52: 

This is a libstdc++ header, so it cannot be a libc++ bug.

It is however a clang bug. See below.

> The issue was discussed here: https://github.com/greg7mdp/sparsepp/issues/61
> where greg7mdp <https://github.com/greg7mdp> suggested that the problem 
> might be a missing `std` in front of the swap() call:
> 

It might resolve the issue but is not a correct solution.

> |void swap(array& __other) 
> noexcept(noexcept(_*std::*_swap(std::declval<_Tp&>(), 
> std::declval<_Tp&>()))) { std::swap_ranges(begin(), end(), 
> __other.begin()); }|
> 
> 

The second, unqualified `swap` is envisioned to involve the Argument 
Dependent Lookup (ADL) and call whatever ADL finds, and if ADL fails to 
find (a most probably user-defined) one, call the one in `std`.

Here clang behaves as if the second `swap` is hidden by the first one 
being declared. This is the bug, because the exception specification is 
part of the function declarator and is before the point of declaration 
of the function being declared, and consequently, shall not make the 
name of the function in question visible.

The use of the qualified name `std::swap` prevents the second `swap` 
from being hidden as described above, but it also forbids ADL so 
user-defined `swap` functions cannot be found at all. This is a minor 
issue if you have had move assignment operators implemented properly, 
nevertheless.

> Please let me know if this is a bug in libc++, or an issue with 
> sparsepp, where the problem manifests.
> 
> Thanks
> Lucas
> 



-- 
Best regards,
LH_Mouse