[v3 PATCH] Implement LWG 2758.

[Jonathan Wakely]

On 16/08/16 12:52 +0100, Jonathan Wakely wrote:
>On 10/08/16 10:50 +0300, Ville Voutilainen wrote:
>>diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
>>index 59f1c64..89e2100 100644
>>--- a/libstdc++-v3/include/bits/basic_string.h
>>+++ b/libstdc++-v3/include/bits/basic_string.h
>>@@ -1227,9 +1227,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>>       *  @param __n   The number of characters to append from the string_view.
>>       *  @return  Reference to this string.
>>       */
>>-      basic_string& append(__sv_type __sv,
>>+      template <typename _Tp,
>>+		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
>>+			    bool> = true>
>>+      basic_string& append(const _Tp& __svt,
>>			   size_type __pos, size_type __n = npos)
>
>I would prefer the enable_if constraint to be on the return type, so
>there is only one template parameter:
>
>     template <typename _Tp>
>       enable_if_t<is_convertible_v<const _Tp&, __sv_type>, basic_string&>
>       append(const _Tp& __svt,
>              size_type __pos, size_type __n = npos)
>
>However these function templates appear to have a design problem, see
>PR 77264.


This patch constrains the new overloads using:

  template<typename _Tp, typename _Res>
    using _If_sv = enable_if_t<
      __and_<is_convertible<const _Tp&, __sv_type>,
	     __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
      _Res>;

so that anything convertible to const _CharT* goes to the same
overload as would have been chosen prior to C++17.

I'm testing this now but won't commit it until it's clear this is a
sensible resolution for LWG 2758.

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