libstdc++: Define __cpp_lib_constexpr_string macro

As noted in r11-1339-gb6ab9ecd550227684643b41e9e33a4d3466724d8 we define
a non-standard __cpp_lib_constexpr_char_traits feature test macro to
indicate support for P0426R1 and P1032R1. At some point last year the
__cpp_lib_constexpr_string macro was retconned to indicate support for
those papers. This adds the new macro (which we didn't previously
define, because it referred to P0980R1 "Making std::string constexpr"
which we don't support).

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__cpp_lib_constexpr_string): Define.
	* include/std/version (__cpp_lib_constexpr_string): Define.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc:
	Check for __cpp_lib_constexpr_string.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
	Likewise.
	* testsuite/21_strings/char_traits/requirements/version.cc: New test.

5 files changed, 41 insertions, 4 deletions

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 3c5309d..41d781c 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -48,11 +48,18 @@
 # include <string_view>
 #endif
 
-
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+#if __cplusplus == 201703L
+// Support P0426R1 changes to char_traits in C++17.
+# define __cpp_lib_constexpr_string 201611L
+#else
+// Also support P1032R1 in C++20 (but not P0980R1 yet).
+# define __cpp_lib_constexpr_string 201811L
+#endif
+
 #if _GLIBCXX_USE_CXX11_ABI
 _GLIBCXX_BEGIN_NAMESPACE_CXX11
   /**
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index d9f6a3c..09e67a4 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -131,6 +131,7 @@
 #define __cpp_lib_clamp 201603
 #if __cplusplus == 201703L // N.B. updated value in C++20
 # define __cpp_lib_constexpr_char_traits 201611L
+# define __cpp_lib_constexpr_string 201611L
 #endif
 #define __cpp_lib_enable_shared_from_this 201603
 #define __cpp_lib_execution 201902L // FIXME: should be 201603L
@@ -219,6 +220,7 @@
 #define __cpp_lib_constexpr_iterator 201811L
 #define __cpp_lib_constexpr_memory 201811L
 #define __cpp_lib_constexpr_numeric 201911L
+#define __cpp_lib_constexpr_string 201811L
 #define __cpp_lib_constexpr_string_view 201811L
 #define __cpp_lib_constexpr_tuple 201811L
 #define __cpp_lib_constexpr_utility 201811L
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc
index 803fb68..32e31e1 100644
--- a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc
@@ -73,12 +73,17 @@ template<typename CT>
     return true;
   }
 
+#ifndef __cpp_lib_constexpr_string
+# error Feature-test macro for constexpr char_traits is missing
+#elif __cpp_lib_constexpr_string < (__cplusplus == 201703 ? 201611 : 201811)
+# error Feature-test macro for constexpr char_traits has the wrong value
+#endif
+
+// We also provide this non-standard macro for P0426R1 (and P1032R1 in C++20).
 #ifndef __cpp_lib_constexpr_char_traits
 # error Feature-test macro for constexpr char_traits is missing
-#elif __cpp_lib_constexpr_char_traits < 201611
+#elif __cpp_lib_constexpr_char_traits != (__cplusplus == 201703 ? 201611 : 201811)
 # error Feature-test macro for constexpr char_traits has the wrong value
-#elif __cpp_lib_constexpr_char_traits > 201611 && __cplusplus == 201703
-# error Feature-test macro for constexpr char_traits has wrong value for C++17
 #endif
 
 static_assert( test_assign<std::char_traits<char>>() );
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc
index 0c1329f..0d1595f 100644
--- a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc
@@ -30,6 +30,13 @@ template<typename CT>
     return s1[0]==char_type{1} && s1[1]==char_type{1} && s1[2]==char_type{2};
   }
 
+#ifndef __cpp_lib_constexpr_string
+# error Feature-test macro for constexpr char_traits is missing
+#elif __cpp_lib_constexpr_string < 201811
+# error Feature-test macro for constexpr char_traits has the wrong value
+#endif
+
+// We also provide this non-standard macro for P0426R1 and P1032R1.
 #ifndef __cpp_lib_constexpr_char_traits
 # error Feature-test macro for constexpr char_traits is missing
 #elif __cpp_lib_constexpr_char_traits != 201811
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/version.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/version.cc
new file mode 100644
index 0000000..6f571f4
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/version.cc
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++17 } }
+
+#include <version>
+
+#ifndef __cpp_lib_constexpr_string
+# error Feature-test macro for constexpr char_traits is missing in <version>
+#elif __cpp_lib_constexpr_string < (__cplusplus == 201703 ? 201611 : 201811)
+# error Feature-test macro for constexpr char_traits has the wrong value in <version>
+#endif
+
+// We also provide this non-standard macro for P0426R1 and P1032R1.
+#ifndef __cpp_lib_constexpr_char_traits
+# error Feature-test macro for constexpr char_traits is missing in <version>
+#elif __cpp_lib_constexpr_char_traits != (__cplusplus == 201703 ? 201611 : 201811)
+# error Feature-test macro for constexpr char_traits has the wrong value in <version>
+#endif