When deduction guides are supported by the compiler (i.e. for C++17 and
later) replace two basic_string constructors by constrained function
templates as required by LWG 3076. In order to ensure that the pre-C++17
non-template constructors are still exported from the shared library
define a macro in src/c++11/string-inst.cc to force the non-template
declarations (this isn't strictly needed yet, because the string
instantiations are compiled with -std=gnu++11, but that is likely to
change).
LWG 3076 basic_string CTAD ambiguity
* doc/xml/manual/intro.xml: Document LWG 3076 change.
* include/bits/basic_string.h
[__cpp_deduction_guides && !_GLIBCXX_DEFINING_STRING_INSTANTIATIONS]
(basic_string(const _CharT*, const _Alloc&)): Turn into a function
template constrained by _RequireAllocator.
(basic_string(size_type, _CharT, const _Alloc&)): Likewise.
* src/c++11/string-inst.cc (_GLIBCXX_DEFINING_STRING_INSTANTIATIONS):
Define.
* testsuite/21_strings/basic_string/cons/char/deduction.cc: Test
deduction
* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc:
Likewise.
Tested powerpc64le-linux, committed to trunk.
commit d4fe0dd3286562ffa26b993c9ee8e80c62762dc6
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Thu Jun 14 20:27:04 2018 +0000
LWG 3076 basic_string CTAD ambiguity
When deduction guides are supported by the compiler (i.e. for C++17 and
later) replace two basic_string constructors by constrained function
templates as required by LWG 3075. In order to ensure that the pre-C++17
non-template constructors are still exported from the shared library
define a macro in src/c++11/string-inst.cc to force the non-template
declarations (this isn't strictly needed yet, because the string
instantiations are compiled with -std=gnu++11, but that is likely to
change).
LWG 3076 basic_string CTAD ambiguity
* doc/xml/manual/intro.xml: Document LWG 3076 change.
* include/bits/basic_string.h
[__cpp_deduction_guides && !_GLIBCXX_DEFINING_STRING_INSTANTIATIONS]
(basic_string(const _CharT*, const _Alloc&)): Turn into a function
template constrained by _RequireAllocator.
(basic_string(size_type, _CharT, const _Alloc&)): Likewise.
* src/c++11/string-inst.cc
(_GLIBCXX_DEFINING_STRING_INSTANTIATIONS):
Define.
* testsuite/21_strings/basic_string/cons/char/deduction.cc: Test
deduction
* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc:
Likewise.
diff --git a/libstdc++-v3/doc/xml/manual/intro.xml
b/libstdc++-v3/doc/xml/manual/intro.xml
index 90d43903def..fea07e2bb5f 100644
--- a/libstdc++-v3/doc/xml/manual/intro.xml
+++ b/libstdc++-v3/doc/xml/manual/intro.xml
@@ -1176,6 +1176,13 @@ requirements of the license of GCC.
they will allow conversions from other types to the value_type.
</para></listitem></varlistentry>
+ <varlistentry xml:id="manual.bugs.dr3076"><term><link
xmlns:xlink="http://www.w3.org/1999/xlink"; xlink:href="&DR;#3076">3076</link>:
+ <emphasis><code>basic_string</code> CTAD ambiguity
+ </emphasis>
+ </term>
+ <listitem><para>Change constructors to constrained templates.
+ </para></listitem></varlistentry>
+
</variablelist>
</section>
diff --git a/libstdc++-v3/include/bits/basic_string.h
b/libstdc++-v3/include/bits/basic_string.h
index 577ad5e2f71..2e6e1c6e5da 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -506,6 +506,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
* @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())
: _M_dataplus(_M_local_data(), __a)
{ _M_construct(__s, __s ? __s + traits_type::length(__s) : __s+npos); }
@@ -516,6 +521,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
* @param __c Character to use.
* @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(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
: _M_dataplus(_M_local_data(), __a)
{ _M_construct(__n, __c); }
diff --git a/libstdc++-v3/src/c++11/string-inst.cc
b/libstdc++-v3/src/c++11/string-inst.cc
index 567d817dd7b..47a1c9af3ab 100644
--- a/libstdc++-v3/src/c++11/string-inst.cc
+++ b/libstdc++-v3/src/c++11/string-inst.cc
@@ -35,6 +35,12 @@
# define _GLIBCXX_USE_CXX11_ABI 1
#endif
+// Prevent the basic_string(const _CharT*, const _Alloc&) and
+// basic_string(size_type, _CharT, const _Alloc&) constructors from being
+// replaced by constrained function templates, so that we instantiate the
+// pre-C++17 definitions.
+#define _GLIBCXX_DEFINING_STRING_INSTANTIATIONS 1
+
#include <string>
// Instantiation configuration.
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 cf6857c6678..fc28467e29b 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
@@ -138,3 +138,20 @@ test05()
std::basic_string s4(sv, 2u, 6u, a);
check_type<std::string>(s4);
}
+
+void
+test06()
+{
+ // LWG 3076 basic_string CTAD ambiguity
+ using namespace std;
+ string s0;
+
+ basic_string s1(s0, 1, 1);
+ check_type<std::string>(s1);
+
+ basic_string s2("cat"sv, 1, 1);
+ check_type<std::string>(s2);
+
+ basic_string s3("cat", 1);
+ check_type<std::string>(s3);
+}
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 ea312ff653e..c40651f13db 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
@@ -97,3 +97,20 @@ test05()
std::basic_string s4(sv, 2u, 6u, a);
check_type<std::wstring>(s4);
}
+
+void
+test06()
+{
+ // LWG 3076 basic_string CTAD ambiguity
+ using namespace std;
+ wstring s0;
+
+ basic_string s1(s0, 1, 1);
+ check_type<std::wstring>(s1);
+
+ basic_string s2(L"cat"sv, 1, 1);
+ check_type<std::wstring>(s2);
+
+ basic_string s3(L"cat", 1);
+ check_type<std::wstring>(s3);
+}
