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

[Jonathan Wakely]

On 20/09/17 17:52 +0100, Jonathan Wakely wrote:
>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>;
>
>

I'm committing this to fix the regression. If the final LWG 2946
resolution does something different we can change it again.

Tested powerpc64le-linux, committed to trunk.

I'll start another test run with all variations ...


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