Cleanup sign checking in safe math code

base/numerics/safe_math_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_NUMERICS_SAFE_MATH_IMPL_H_
 #define BASE_NUMERICS_SAFE_MATH_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 28 matching lines @@
 
 template <typename Numeric>
 struct UnsignedOrFloatForSize<Numeric, false, true> {
   using type = Numeric;
 };
 
 // Helper templates for integer manipulations.
 
 template <typename T>
 constexpr bool HasSignBit(T x) {
   // Cast to unsigned since right shift on signed is undefined.

[scottmg, 2016/12/08 21:05:02]

You're not casting to unsigned.

-  return !!(static_cast<typename std::make_unsigned<T>::type>(x) >>
-            PositionOfSignBit<T>::value);
+  return static_cast<typename std::make_signed<T>::type>(x) <
+         static_cast<typename std::make_signed<T>::type>(0);
 }
 
 // This wrapper undoes the standard integer promotions.
 template <typename T>
 constexpr T BinaryComplement(T x) {
   return static_cast<T>(~x);
 }
 
 // Probe for builtin math overflow support on Clang and version check on GCC.
 #if defined(__has_builtin)
@@ skipping 668 matching lines @@
   using math = M<typename UnderlyingType<L>::type,
                  typename UnderlyingType<R>::type,
                  void>;
   using type = typename math::result_type;
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_NUMERICS_SAFE_MATH_IMPL_H_