Tested on x86_64-linux-gnu, powerpc64le-linux-gnu, aarch64-linux-gnu, arm-
linux-gnueabihf

OK for trunk and backports?

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


Avoid
-Wnarrowing in C code;
-Wtautological-compare in unconditional static_assert (necessary for
faking a dependency on a template parameter)

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

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h: Ignore -Wnarrowing for
	arm_neon.h.
	(__int_for_sizeof): Replace tautological compare with checking
	for invalid template parameter value.
	* include/experimental/bits/simd_builtin.h (__extract_part):
	Remove tautological compare by combining two static_assert.
---
 libstdc++-v3/include/experimental/bits/simd.h         | 8 +++++++-
 libstdc++-v3/include/experimental/bits/simd_builtin.h | 3 +--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/
include/experimental/bits/simd.h
index 03c2e17a326..6ef9c955cfa 100644
--- a/libstdc++-v3/include/experimental/bits/simd.h
+++ b/libstdc++-v3/include/experimental/bits/simd.h
@@ -44,7 +44,12 @@
 #if _GLIBCXX_SIMD_X86INTRIN
 #include <x86intrin.h>
 #elif _GLIBCXX_SIMD_HAVE_NEON
+#pragma GCC diagnostic push
+// narrowing conversion of '__a' from 'uint64_t' {aka 'long long unsigned 
int'} to
+//   'int64x1_t' {aka 'long long int'} [-Wnarrowing]
+#pragma GCC diagnostic ignored "-Wnarrowing"
 #include <arm_neon.h>
+#pragma GCC diagnostic pop
 #endif
 #if _GLIBCXX_SIMD_HAVE_SVE
 #include <arm_sve.h>
@@ -598,6 +603,7 @@ struct __is_bitmask
   constexpr auto
   __int_for_sizeof()
   {
+    static_assert(_Bytes > 0);
     if constexpr (_Bytes == sizeof(int))
       return int();
   #ifdef __clang__
@@ -663,7 +669,7 @@ struct _Ip
 	return _Ip{};
       }
     else
-      static_assert(_Bytes != _Bytes, "this should be unreachable");
+      static_assert(_Bytes == 0, "this should be unreachable");
   }
 #pragma GCC diagnostic pop
 
diff --git a/libstdc++-v3/include/experimental/bits/simd_builtin.h b/libstdc+
+-v3/include/experimental/bits/simd_builtin.h
index af0c4886108..49c7c7e1c70 100644
--- a/libstdc++-v3/include/experimental/bits/simd_builtin.h
+++ b/libstdc++-v3/include/experimental/bits/simd_builtin.h
@@ -278,8 +278,7 @@ __extract_part(const _SimdWrapper<_Tp, _Np> __x)
   __extract_part(const _SimdWrapper<bool, _Np> __x)
   {
     static_assert(_Combine == 1, "_Combine != 1 not implemented");
-    static_assert(__have_avx512f && _Np == _Np);
-    static_assert(_Total >= 2 && _Index + _Combine <= _Total && _Index >= 0);
+    static_assert(__have_avx512f && _Total >= 2 && _Index + _Combine <= 
_Total && _Index >= 0);
     return __x._M_data >> (_Index * _Np / _Total);
   }
 
-- 
──────────────────────────────────────────────────────────────────────────
 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 stdₓ::simd
──────────────────────────────────────────────────────────────────────────