[Bug libstdc++/116399] New: C++26 text_encoding​::​aliases_view is not default-constructible

[Bug libstdc++/116399] New: C++26 text_encoding​::​aliases_view is not default-constructible

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

            Bug ID: 116399
           Summary: C++26 text_encoding​::​aliases_view is not
                    default-constructible
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

text.encoding.aliases] indicates that text_encoding::aliases_view is an empty
struct, which suggests it is default-constructible.

However, the private custom constructors in the libstdc++ implementation make
it not so:

#include <text_encoding>

std::text_encoding::aliases_view av{};

https://godbolt.org/z/MahWfeW81

Not sure if this is LWG-worthy.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The working draft says:

  text_encoding::aliases_view models copyable, ranges::view,
ranges::random_access_range,
  and ranges::borrowed_range.

It specifically says copyable and not semiregular, which suggests to me that it
isn't required to model semiregular, so isn't required to be
default-initializable.

I think libstdc++ is correct, and the working draft should be clarified.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |corentinjabot at gmail dot com

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
CC Corentin for confirmation of the intent.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[corentinjabot at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #3 from corentinjabot at gmail dot com ---
Yes, libstdc++ is correct.
Not sure what one would do with a default constructed alias aliases_view

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks! I'll submit an LWG issue to add something saying there's an unspecified
constructor, or just say explicitly that it's not default constructible.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #5 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> Thanks! I'll submit an LWG issue to add something saying there's an
> unspecified constructor, or just say explicitly that it's not default
> constructible.

Not sure it's worth specifying whether aliases_view should or should not be
common_range, although currently, libstdc++ implementations are
non-common_range.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's discussed in P1885.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
"Note that this view is not common_range because it can be implemented more
efficiently
without that requirement, and, being copyable, it can be adapted into one."

and

"We found that aliases can be efficiently stored and looked up in a sorted list
of alias/mib pairs. Making a common_range of aliases_view would force an
implementation to find the end of the list of aliases for a particular
encoding, which is slightly efficient than what is possible, so this is not
proposed. Mostly, we found little use for it to be a common_range."

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #8 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #7)
> "Note that this view is not common_range because it can be implemented more
> efficiently
> without that requirement, and, being copyable, it can be adapted into one."
> 
> and
> 
> "We found that aliases can be efficiently stored and looked up in a sorted
> list of alias/mib pairs. Making a common_range of aliases_view would force
> an implementation to find the end of the list of aliases for a particular
> encoding, which is slightly efficient than what is possible, so this is not
> proposed. Mostly, we found little use for it to be a common_range."

If so, why not just explicitly specify that end() returns
std::default_sentinel? libstdc++'s _Sentinel is just an empty class, so there
is no need to introduce an additional class IMO.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Currently aliases_view is allowed to be a common range, but not required to be.

If we specify that its sentinel type is std::default_sentinel, that would
require it to be a non-common range. Why would we want that?

If an implementation decides to use a different design for the aliases_view
which is more efficient with a common range, your suggestion would forbid them
from using that.

So I don't think we should require that. Whether libstdc++ should make that
change is a different matter.

I guess we could also defined the sentinel as an enum type, which might be a
little less expensive than a new class type. Or just use default_sentinel_t.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #10 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #9)
> Currently aliases_view is allowed to be a common range, but not required to
> be.
> 
> If we specify that its sentinel type is std::default_sentinel, that would
> require it to be a non-common range. Why would we want that?
> 
> If an implementation decides to use a different design for the aliases_view
> which is more efficient with a common range, your suggestion would forbid
> them from using that.
> 
> So I don't think we should require that. Whether libstdc++ should make that
> change is a different matter.
> 
> I guess we could also defined the sentinel as an enum type, which might be a
> little less expensive than a new class type. Or just use default_sentinel_t.

But we should at least make it clear in the standard whether aliases_view is
common range is *unspecified*, right?
Currently, the return type of begin()/end() is *implementation defined*, which
seems confusing because the two types appear to be the same as we indicated for
iterators in the [container], and containers are always common ranges.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't agree. Containers do:

class X {
  using iterator = <implementation-defined>;
  iterator begin();
  iterator end();
};

So clearly the type is the same.

That's not how text_encoding::aliases_view is specified.

The standard doesn't say begin() and end() return the same type, and it doesn't
say aliases_view models common_range. So it's already unspecified.

[Bug libstdc++/116399] C++26 text_encoding​::​aliases_view is not default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116399

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Implementation-defined means the implementation must define it (and document
it).

If the implementation says the types are different, then they're different, and
it's not a common range.