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

[Jonathan Wakely]

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.krueg...@gmail.com>:

2017-07-28 22:29 GMT+02:00 Daniel Krügler <daniel.krueg...@gmail.com>:

2017-07-28 22:25 GMT+02:00 Tim Song <t.canens....@gmail.com>:

On Fri, Jul 28, 2017 at 4:10 PM, Daniel Krügler
<daniel.krueg...@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!

We also need this tweak, to account for the fact that the old
std::string has this signature:

     basic_string& replace(iterator __i1, iterator __i2,
                            initializer_list<_CharT> __l)


Tested powerpc64le-linux, committed to trunk.



commit c435fd7ed919ae54fc8f730844c7466822787ff8
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Sep 11 17:29:34 2017 +0100

    Adjust test to pass with old std::string
    
            * testsuite/21_strings/basic_string/lwg2946.cc: Adjust for
            compatibility with old COW std::string.

diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/lwg2946.cc b/libstdc++-v3/testsuite/21_strings/basic_string/lwg2946.cc
index 74d5a5c89a7..fe1f15553fb 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/lwg2946.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/lwg2946.cc
@@ -29,7 +29,7 @@ int main()
   s.assign({"abc", 1});
   s.insert(0, {"abc", 1});
   s.replace(0, 1, {"abc", 1});
-  s.replace(s.cbegin(), s.cbegin(), {"abc", 1});
+  s.replace(s.begin(), s.begin(), {"abc", 1});
   s.find({"abc", 1});
   s.rfind({"abc", 1});
   s.find_first_of({"abc", 1});