Re: [PATCH] PR libstdc++/79162 ambiguity in string assignment due to string_view overload (LWG 2946)

[Jonathan Wakely <jwakely at redhat dot com>]

On 04/09/17 16:48 +0100, Jonathan Wakely wrote:
> On 30/07/17 15:01 +0200, Daniel Krügler wrote:
> > 2017-07-28 22:40 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:
> > > 2017-07-28 22:29 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:
> > > > 2017-07-28 22:25 GMT+02:00 Tim Song <t.canens.cpp@gmail.com>:
> > > > > On Fri, Jul 28, 2017 at 4:10 PM, Daniel Krügler
> > > > > <daniel.kruegler@gmail.com> wrote:
> > > > > > +      // Performs an implicit conversion from _Tp to __sv_type.
> > > > > > +      template<typename _Tp>
> > > > > > +        static __sv_type _S_to_string_view(const _Tp& __svt)
> > > > > > +        {
> > > > > > +          return __svt;
> > > > > > +        }
> > > > >
> > > > > I might have gone for
> > > > >
> > > > > +        static __sv_type _S_to_string_view(__sv_type __svt) noexcept
> > > > > +        {
> > > > > +          return __svt;
> > > > > +        }
> > > > >
> > > > > With that, we can also use noexcept(_S_to_string_view(__t)) to make up
> > > > > for the absence of is_nothrow_convertible (basically the same thing I
> > > > > did in LWG 2993's PR).
> > > >
> > > > Agreed, that makes very much sense. I will adjust the P/R, but before
> > > > I resubmit I would like to get feedback whether the other two compare
> > > > functions also should become conditionally noexcept.
> > >
> > > Locally I have now performed the sole change of the _S_to_string_view
> > > declaration getting rid of the template, but would also like to gather
> > > feedback from the maintainers whether I should also change the form of
> > > the conditional noexcept to use the expression
> > >
> > > noexcept(_S_to_string_view(__t))
> > >
> > > instead of the current
> > >
> > > is_same<_Tp, __sv_type>::value
> > >
> > > as suggested by Tim Song.
> > >
> > > I'm asking also, because I have a paper proposing to standardize
> > > is_nothrow_convertible submitted for the upcoming C++ mailing - This
> > > would be one of the first applications in the library ;-)
> >
> > A slightly revised patch update: It replaces the _S_to_string_view
> > template by a simpler _S_to_string_view function as of Tim Song's
> > suggestion, but still uses the simplified noexcept specification
> > deferring it to a future application case for is_nothrow_convertible.
> > Furthermore now all three compare function templates are now
> > (conditionally) noexcept by an (off-list) suggestion from Jonathan
> > Wakely.
>
> I've committed this, after some whitespace fixes and testing.
>
> Thanks!

This change causes two regressions in C++17 mode, see
https://gcc.gnu.org/ml/gcc-testresults/2017-09/msg01674.html

FAIL: 21_strings/basic_string/cons/char/moveable2.cc execution test
FAIL: 21_strings/basic_string/cons/wchar_t/moveable2.cc execution test

Here's a reduced version of that test, which passes in C++14 and fails
in C++17:

#include <string>
#include <assert.h>

class tstring : public std::string
{
public:
 tstring() : std::string() {}
 tstring(tstring&& s) : std::string(std::move(s)) {}
};

int main()
{
 tstring b;
 b.push_back('1');
 tstring c(std::move(b));
 assert( c.size() == 1 && c[0] == '1' );
 assert( b.size() == 0 );
}

The second assertion fails, because this mem-initializer:

 tstring(tstring&& s) : std::string(std::move(s)) {}

now prefers to use the new constructor:

 basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())

because tstring is convertible to string_view.

This turns a non-allocating move into an allocating copy.