This never showed up as an issue because it's an internal header and 
implicitly guarded by bits/simd.h.

OK for trunk? Any reason to backport?

--------------------- 8< ------------------

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	* include/experimental/bits/numeric_traits.h: Add include guard.
---
 libstdc++-v3/include/experimental/bits/numeric_traits.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/include/experimental/bits/numeric_traits.h b/
libstdc++-v3/include/experimental/bits/numeric_traits.h
index b195de7c3db..8717f684d96 100644
--- a/libstdc++-v3/include/experimental/bits/numeric_traits.h
+++ b/libstdc++-v3/include/experimental/bits/numeric_traits.h
@@ -22,6 +22,9 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#ifndef _GLIBCXX_EXPERIMENTAL_BITS_NUMERIC_TRAITS_H
+#define _GLIBCXX_EXPERIMENTAL_BITS_NUMERIC_TRAITS_H
+
 #include <type_traits>
 
 namespace std {
@@ -565,3 +568,4 @@ struct __has_iec559_behavior<__signaling_NaN, long 
double>
 #endif // __FINITE_MATH_ONLY__
 
 } // namespace std
+#endif  // _GLIBCXX_EXPERIMENTAL_BITS_NUMERIC_TRAITS_H
-- 
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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 stdₓ::simd
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