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

[Jonathan Wakely]

On 20/09/17 16:36 +0100, Jonathan Wakely wrote:
>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.

This patch fixes the failure above, I'm testing it now.

--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -115,6 +115,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       template<typename _Tp, typename _Res>
        using _If_sv = enable_if_t<
          __and_<is_convertible<const _Tp&, __sv_type>,
+                __not_<is_convertible<const _Tp*, const basic_string*>>,
                 __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
          _Res>;