[libcxx-dev] Providing custom template specialization for std::optional?

[Louis Dionne via libcxx-dev]

> On Mar 7, 2019, at 00:39, Eric Fiselier via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> 
> 
> On Wed, Mar 6, 2019 at 5:41 PM Владимир Гончаров via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
> Hello!
> 
> I'm implementing a specialization for std::optional for my program-defined type.
> 
> Can you explain what your specialization does differently? And why it's needed?
>  
> As I understand [namespace.std], nothing forbids me from doing that.
> 
> I agree that you've interpreted the standard correctly. But I think the standard is being too permissive here.
> Implementing a complex component may necessitate knowledge of the internals. `<variant>` is a good example.
> 
> I would like to know what other library maintainers think.

I think it is a bug in the Standard that we don't explicitly state which templates are safe to specialize, and prohibit doing so by default. I would not try to do anything special to support this in libc++ and I would instead fix the Standard.

Louis

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