[Bug libstdc++/90704] [LWG 3430] filesystem::path overloads for file streams are not conforming

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90704

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jörn Heusipp from comment #5)
> > Status: SUSPENDED
> 
> Well, coming from bug 95642, which has been marked as a duplicate of this
> bug, this is *very* disappointing.
> 
> 
> The C++17 standard absolutely clearly specifies that the constructor is
> overloaded for std::filesystem::path. libstdc++'s SFINAE implementation is
> incompatible because it fails for user-defined types that are implicitly
> convertible to std::filesystem::path.

And also for string_view, because that's convertible to filesystem::path, and
that is the topic of the issue reported against the standard.

> In order to at all be able to sensibly use libstdc++ std::filesystem::path
> in my codebase, I would have to put this awful work-around (making tons of
> assumptions about libstdc++'s enable_if expression) into my own type
> mpt::PathString:

Or you can just convert it to filesystem::path explicitly before passing it to
fstream members.


> and just pray and hope that libstdc++ will not change its non-conforming
> SFINAE-using implementation details to some other non-conforming
> implementation details in some future version.

Or just convert to filesystem::path explicitly.

> Can you guarantee, that you wont ever change your non-conforming SFINAE
> implementation whatsoever at all? If not, please just fix it. The standard
> is absolutely clear here.

The standard is flawed and no longer follows its own design. I want to get some
clarity about the intended design.

> After having said all this, I am not even sure anymore that this bug report
> here actually is a duplicate of my bug 95642. I am concerned about implicit
> conversion to std::filesystem::path not working, while this bug report is
> concerned about std::string_view not working. Please consider re-opening bug
> 95642.

They're exactly the same.

string_view is expected to work by converting to filesystem::path, which is the
same as your type.