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).

Backport from mainline
2018-06-16  Jonathan Wakely  <jwakely@redhat.com>

	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.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-8-branch@262384 138bc75d-0d04-0410-961f-82ee72b054a4

6 files changed, 74 insertions, 0 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index b5815c6f53ff..c7ca508cc1e0 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,6 +1,23 @@
 2018-07-04  Jonathan Wakely  <jwakely@redhat.com>
 
 	Backport from mainline
+	2018-06-16  Jonathan Wakely  <jwakely@redhat.com>
+
+	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.
+
+	Backport from mainline
 	2018-06-14  Jonathan Wakely  <jwakely@redhat.com>
 
 	LWG 3075 basic_string needs deduction guides from basic_string_view
diff --git a/libstdc++-v3/doc/xml/manual/intro.xml b/libstdc++-v3/doc/xml/manual/intro.xml
index 38b6eb36477b..5d20778c0c60 100644
--- a/libstdc++-v3/doc/xml/manual/intro.xml
+++ b/libstdc++-v3/doc/xml/manual/intro.xml
@@ -1160,6 +1160,13 @@ requirements of the license of GCC.
     <listitem><para>Add noexcept.
     </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 577ad5e2f712..2e6e1c6e5da3 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 567d817dd7b8..47a1c9af3ab1 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 cf6857c6678d..fc28467e29b2 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 ea312ff653eb..c40651f13dbd 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);
+}