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).
	* testsuite/21_strings/basic_string/cons/char/deduction.cc:
	Remove dg-skip-if.
	* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc:
	Likewise.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index a9fe09f2069aad95c5ed49af41ed832b868dc927..4b3722bdbf1831b435970918a5d28a038c44a78b 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3612,12 +3612,19 @@ _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).
        */
+#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());
+
       /**
        *  @brief  Construct string as multiple characters.
        *  @param  __n  Number of characters.

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 6484ed4345335e00b941f791f31731741eb2aef5..d05c4b776c32031af1076a00abda758ba38a534c 100644 (file)
--- 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 373b2b24bdd9affd4dcb46aa22ca6ba05103c2ef..1773be28e3778b4a9dd2b4690f823d6b6cf345f5 100644 (file)
--- 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>