[gcc r12-1942] libstdc++: Make <experimental/simd> depend on C++17

[Jonathan Wakely]

https://gcc.gnu.org/g:36adced3b6b7cae17d7b42e4573bd0f4194b1f70

commit r12-1942-g36adced3b6b7cae17d7b42e4573bd0f4194b1f70
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 30 21:35:51 2021 +0100

    libstdc++: Make <experimental/simd> depend on C++17
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/experimental/simd: Do not define anything pre-C++17.

Diff:
---
 libstdc++-v3/include/experimental/simd | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libstdc++-v3/include/experimental/simd b/libstdc++-v3/include/experimental/simd
index da2d0479cf3..beccbc14fa5 100644
--- a/libstdc++-v3/include/experimental/simd
+++ b/libstdc++-v3/include/experimental/simd
@@ -33,6 +33,8 @@
 #ifndef _GLIBCXX_EXPERIMENTAL_SIMD
 #define _GLIBCXX_EXPERIMENTAL_SIMD
 
+#if __cplusplus >= 201703L
+
 #define __cpp_lib_experimental_parallel_simd 201803
 
 #pragma GCC diagnostic push
@@ -66,5 +68,6 @@
 
 #pragma GCC diagnostic pop
 
+#endif // C++17
 #endif // _GLIBCXX_EXPERIMENTAL_SIMD
 // vim: ft=cpp