[PATCH] Only define __cpp_lib_constexpr_char_traits for C++17

[Jonathan Wakely]

This macro should not be defined unconditionally.

The macro isn't official (it was proposed in P0426R1 but doesn't
appear in SD-6 or the drafts, or the C++2a working paper) so I haven't
added any tests. Ideally we would have negative tests for each feature
test macro, to make sure they aren't defined when the feature isn't
available. I've done a review and this seems to be the only problem.

        * include/bits/char_traits.h (__cpp_lib_constexpr_char_traits): Only
        define for C++17 and above.

Tested powerpc64le-linux, committed to trunk.


commit 8e1d107e8d81c7aa5d74246d00cdd791cbd4beb2
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Jun 15 16:31:23 2018 +0100

    Only define __cpp_lib_constexpr_char_traits for C++17
    
            * include/bits/char_traits.h (__cpp_lib_constexpr_char_traits): Only
            define for C++17 and above.

diff --git a/libstdc++-v3/include/bits/char_traits.h 
b/libstdc++-v3/include/bits/char_traits.h
index 51cd321b8ae..7cc7c74e8fe 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -143,8 +143,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return !eq_int_type(__c, eof()) ? __c : to_int_type(char_type()); }
     };
 
-#define __cpp_lib_constexpr_char_traits 201611
-
   template<typename _CharT>
     _GLIBCXX14_CONSTEXPR int
     char_traits<_CharT>::
@@ -216,7 +214,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
+#define __cpp_lib_constexpr_char_traits 201611
+
   /**
    *  @brief Determine whether the characters of a NULL-terminated
    *  string are known at compile time.
@@ -299,7 +299,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR int
       compare(const char_type* __s1, const char_type* __s2, size_t __n)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__builtin_constant_p(__n)
            && __constant_char_array_p(__s1, __n)
            && __constant_char_array_p(__s2, __n))
@@ -313,7 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR size_t
       length(const char_type* __s)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__constant_string_p(__s))
          return __gnu_cxx::char_traits<char_type>::length(__s);
 #endif
@@ -323,7 +323,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR const char_type*
       find(const char_type* __s, size_t __n, const char_type& __a)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__builtin_constant_p(__n)
            && __builtin_constant_p(__a)
            && __constant_char_array_p(__s, __n))
@@ -408,7 +408,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR int
       compare(const char_type* __s1, const char_type* __s2, size_t __n)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__builtin_constant_p(__n)
            && __constant_char_array_p(__s1, __n)
            && __constant_char_array_p(__s2, __n))
@@ -423,7 +423,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR size_t
       length(const char_type* __s)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__constant_string_p(__s))
          return __gnu_cxx::char_traits<char_type>::length(__s);
        else
@@ -434,7 +434,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static _GLIBCXX17_CONSTEXPR const char_type*
       find(const char_type* __s, size_t __n, const char_type& __a)
       {
-#if __cplusplus > 201402
+#if __cplusplus >= 201703L
        if (__builtin_constant_p(__n)
            && __builtin_constant_p(__a)
            && __constant_char_array_p(__s, __n))