[gcc r10-9779] libstdc++: Define __cpp_lib_constexpr_string macro

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:8cfa99066ffe9f4d748634d294758dc27ef149ce

commit r10-9779-g8cfa99066ffe9f4d748634d294758dc27ef149ce
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 28 15:56:04 2021 +0100

    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.
    
    (cherry picked from commit 3da80ed7efd582575e7850a403ce693ec882d087)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h                 |  9 ++++++++-
 libstdc++-v3/include/std/version                         |  2 ++
 .../requirements/constexpr_functions_c++17.cc            | 11 ++++++++---
 .../requirements/constexpr_functions_c++20.cc            |  7 +++++++
 .../21_strings/char_traits/requirements/version.cc       | 16 ++++++++++++++++
 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 f36bd6a4fe6..f51ec6b60cf 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 1cb2e4edb75..a5bfc72a001 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
@@ -207,6 +208,7 @@
 #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 55dcba48db6..ebde98be20e 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 63c7c9cbb57..2c1cf3d2432 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 00000000000..6f571f48010
--- /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