[gcc r10-8922] libstdc++: Implement LWG 3706 for COW strings

[gcc r10-8922] libstdc++: Implement LWG 3706 for COW strings

[Jonathan Wakely]

https://gcc.gnu.org/g:a109beccd84bf255d1b298d8c09d88022d06d4f3

commit r10-8922-ga109beccd84bf255d1b298d8c09d88022d06d4f3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 14 11:52:26 2020 +0100

    libstdc++: Implement LWG 3706 for COW strings
    
    The basic_string deduction guides are defined for the old ABI, but the
    tests are currently disabled. This is because a single case fails when
    using the old ABI, which is just because LWG 3706 isn't implemented for
    the old ABI. That can be done easily, and the tests can be enabled.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
            (basic_string(const _CharT*, const _Alloc&)): Constrain to
            require an allocator-like type to fix CTAD ambiguity (LWG 3706).
            Define inline.
            * include/bits/basic_string.tcc [!_GLIBCXX_USE_CXX11_ABI]
            (basic_string(const _CharT*, const _Alloc&)): Remove non-inline
            definition.
            * testsuite/21_strings/basic_string/cons/char/deduction.cc:
            Remove dg-skip-if.
            * testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc:
            Likewise.
    
    (cherry picked from commit dc38e255242192303ae463a913c060b426eb06c0)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h                     | 12 +++++++++++-
 libstdc++-v3/include/bits/basic_string.tcc                   |  8 --------
 .../testsuite/21_strings/basic_string/cons/char/deduction.cc |  1 -
 .../21_strings/basic_string/cons/wchar_t/deduction.cc        |  1 -
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index bc0c256b65e..bbab7b9d4d2 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3605,12 +3605,22 @@ _GLIBCXX_END_NAMESPACE_CXX11
        */
       basic_string(const _CharT* __s, size_type __n,
 		   const _Alloc& __a = _Alloc());
+
       /**
        *  @brief  Construct string as copy of a C string.
        *  @param  __s  Source C string.
        *  @param  __a  Allocator to use (default is default allocator).
        */
-      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
+#if __cpp_deduction_guides && ! defined _GLIBCXX_DEFINING_STRING_INSTANTIATIONS
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3076. basic_string CTAD ambiguity
+      template<typename = _RequireAllocator<_Alloc>>
+#endif
+      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
+      : _M_dataplus(_S_construct(__s, __s ? __s + traits_type::length(__s) :
+                                 __s + npos, __a), __a)
+      { }
+
       /**
        *  @brief  Construct string as multiple characters.
        *  @param  __n  Number of characters.
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index e370f439390..204a140a735 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -659,14 +659,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : _M_dataplus(_S_construct(__s, __s + __n, __a), __a)
     { }
 
-  // TBD: DPG annotate
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>::
-    basic_string(const _CharT* __s, const _Alloc& __a)
-    : _M_dataplus(_S_construct(__s, __s ? __s + traits_type::length(__s) :
-			       __s + npos, __a), __a)
-    { }
-
   template<typename _CharT, typename _Traits, typename _Alloc>
     basic_string<_CharT, _Traits, _Alloc>::
     basic_string(size_type __n, _CharT __c, const _Alloc& __a)
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/deduction.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/deduction.cc
index 6484ed43453..d05c4b776c3 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/deduction.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/deduction.cc
@@ -17,7 +17,6 @@
 
 // { dg-options "-std=gnu++17" }
 // { dg-do compile { target c++17 } }
-// { dg-xfail-if "COW string missing deduction guides" { ! cxx11-abi } }
 
 #include <string>
 #include <testsuite_iterators.h>
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc
index 373b2b24bdd..1773be28e37 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc
@@ -17,7 +17,6 @@
 
 // { dg-options "-std=gnu++17" }
 // { dg-do compile { target c++17 } }
-// { dg-xfail-if "COW string missing deduction guides" { ! cxx11-abi } }
 
 #include <string>
 #include <testsuite_iterators.h>