Fix left shift of negative value caused by integral promotion

include/rapidcheck/detail/Utility.h

@@ -97,14 +97,17 @@ inline uint64_t avalanche(uint64_t x) {
 template <typename T>
 constexpr T bitMask(int nbits) {
   using UT = typename std::make_unsigned<T>::type;
+  using UTP = typename std::common_type<UT, unsigned>::type;
   // There are two pieces of undefined behavior we're avoiding here,
   //   1. Shifting past the width of a type (ex `<< 32` against an `int32_t`)
   //   2. Shifting a negative operand (which `~0` is for all signed types)
   // First we branch to avoid shifting the past the width of the type, then
   // (assuming we are shifting, and aren't just returning `~0`) we cast `0`
   // to an explicitly unsigned type before performing bitwise NOT and shift.
+  // We're ensuring the target type is as big as unsigned, otherwise it will
+  // be promoted to int before bitwise NOT, producing a negative value.
   return nbits < std::numeric_limits<UT>::digits ?
-         ~T(~UT(0) << nbits)                     :
+         ~T(~UTP(0) << nbits)                    :
          ~T(0);
 }