[gcc r10-9045] libstdc++: Update value of __cpp_lib_constexpr_char_traits for C++20

[Jonathan Wakely]

https://gcc.gnu.org/g:6e4c7cc8d90317a8e706fe35db0d4290367474e7

commit r10-9045-g6e4c7cc8d90317a8e706fe35db0d4290367474e7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jun 15 14:31:26 2020 +0100

    libstdc++: Update value of __cpp_lib_constexpr_char_traits for C++20
    
    Although not required by SD-6 or the C++20 draft, we define the macro
    __cpp_lib_constexpr_char_traits to indicate support for P0432R1. This
    updates the value in C++20 mode for the P1032R1 changes to char_traits.
    
            * include/bits/char_traits.h (__cpp_lib_constexpr_char_traits):
            Update value for C++20.
            * include/std/version (__cpp_lib_constexpr_char_traits): Likewise.
            * testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc:
            Update expected value.
    
    (cherry picked from commit b6ab9ecd550227684643b41e9e33a4d3466724d8)

Diff:
---
 libstdc++-v3/include/bits/char_traits.h                          | 9 ++++++++-
 libstdc++-v3/include/std/version                                 | 6 ++++--
 .../char_traits/requirements/constexpr_functions_c++17.cc        | 4 +++-
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index c6da184e4bd..c2267fa2823 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -236,7 +236,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus >= 201703L
-#define __cpp_lib_constexpr_char_traits 201611
+
+#if __cplusplus == 201703L
+// Unofficial macro indicating P0426R1 support
+# define __cpp_lib_constexpr_char_traits 201611L
+#else
+// Also support P1032R1 in C++20
+# define __cpp_lib_constexpr_char_traits 201811L
+#endif
 
   /**
    *  @brief Determine whether the characters of a NULL-terminated
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index e899a1e6d99..758b121571b 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -127,7 +127,7 @@
 #define __cpp_lib_boyer_moore_searcher 201603
 #define __cpp_lib_chrono 201611
 #define __cpp_lib_clamp 201603
-#define __cpp_lib_constexpr_char_traits 201611
+#define __cpp_lib_constexpr_char_traits 201611L
 #define __cpp_lib_enable_shared_from_this 201603
 #define __cpp_lib_execution 201902L // FIXME: should be 201603L
 #define __cpp_lib_filesystem 201703
@@ -191,16 +191,18 @@
 
 #if _GLIBCXX_HOSTED
 #undef __cpp_lib_array_constexpr
+#undef __cpp_lib_constexpr_char_traits
 #define __cpp_lib_array_constexpr 201811L
 #define __cpp_lib_assume_aligned 201811L
 #define __cpp_lib_bind_front 201907L
 // FIXME: #define __cpp_lib_execution 201902L
 #define __cpp_lib_integer_comparison_functions 202002L
 #define __cpp_lib_constexpr_algorithms 201806L
+#define __cpp_lib_constexpr_char_traits 201811L
 #define __cpp_lib_constexpr_complex 201711L
 #define __cpp_lib_constexpr_dynamic_alloc 201907L
 #define __cpp_lib_constexpr_functional 201907L
-# define __cpp_lib_constexpr_iterator 201811L
+#define __cpp_lib_constexpr_iterator 201811L
 #define __cpp_lib_constexpr_memory 201811L
 #define __cpp_lib_constexpr_numeric 201911L
 #define __cpp_lib_constexpr_string_view 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 7cb950b89f5..55dcba48db6 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
@@ -75,8 +75,10 @@ template<typename CT>
 
 #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 < 201611
 # 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>>() );