[committed] libstdc++: Check feature test macro for robust_nonmodifying_seq_ops

[Jonathan Wakely]

We should check the relevant feature test macro instead of just the
value of __cplusplus.

Also add a comment explaining why the __cplusplus check guarding
__sample *can't* be changed to check __glibcxx_sample (because __sample
is also used in C++14 by std::experimental::sample, not only by C++17
std::sample).

libstdc++-v3/ChangeLog:

        * include/bits/stl_algo.h: Check robust_nonmodifying_seq_ops
        feature test macro instead of checking __cplusplus value. Add
        comment to another __cplusplus check.
        * include/bits/stl_algobase.h: Add comment to #endif.
---

Tested powerpc64le-linux.

Pushed to trunk.

 libstdc++-v3/include/bits/stl_algo.h     | 6 +++---
 libstdc++-v3/include/bits/stl_algobase.h | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_algo.h 
b/libstdc++-v3/include/bits/stl_algo.h
index 78c63e79a279..b296e12cf0f8 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -3516,7 +3516,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
                                   __gnu_cxx::__ops::__iter_comp_iter(__pred));
     }
 
-#if __cplusplus > 201103L
+#if __glibcxx_robust_nonmodifying_seq_ops // C++ >= 14
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
   template<typename _ForwardIterator1, typename _ForwardIterator2,
@@ -3635,7 +3635,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
       return std::__is_permutation(__first1, __last1, __first2, __last2,
                                   __gnu_cxx::__ops::__iter_comp_iter(__pred));
     }
-#endif // C++14
+#endif // __glibcxx_robust_nonmodifying_seq_ops
 
 #ifdef  __glibcxx_clamp // C++ >= 17
   /**
@@ -5828,7 +5828,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     }
 #endif // C++11
 
-#if __cplusplus >= 201402L
+#if __cplusplus >= 201402L // C++17 std::sample and C++14 experimental::sample
   /// Reservoir sampling algorithm.
   template<typename _InputIterator, typename _RandomAccessIterator,
            typename _Size, typename _UniformRandomBitGenerator>
diff --git a/libstdc++-v3/include/bits/stl_algobase.h 
b/libstdc++-v3/include/bits/stl_algobase.h
index 820091aee2dc..022668076138 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -2090,7 +2090,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       return _GLIBCXX_STD_A::__mismatch(__first1, __last1, __first2, __last2,
                             __gnu_cxx::__ops::__iter_comp_iter(__binary_pred));
     }
-#endif
+#endif // __glibcxx_robust_nonmodifying_seq_ops
 
 _GLIBCXX_END_NAMESPACE_ALGO
 
-- 
2.51.0