Fix signed integer UB in random.

Eigen/src/Core/MathFunctions.h

diff --git a/Eigen/src/Core/MathFunctions.h b/Eigen/src/Core/MathFunctions.h
index a0e9d1a..1bb57bb 100644
--- a/Eigen/src/Core/MathFunctions.h
+++ b/Eigen/src/Core/MathFunctions.h
@@ -881,7 +881,9 @@
       // if the random draw is outside [0, range), try again (rejection sampling)
       // in the worst-case scenario, the probability of rejection is: 1/2 - 1/2^numRandomBits < 50%
     } while (randomBits >= range);
-    Scalar result = x + static_cast<Scalar>(randomBits);
+    // Avoid overflow in the case where `x` is negative and there is a large range so
+    // `randomBits` would also be negative if cast to `Scalar` first.
+    Scalar result = static_cast<Scalar>(static_cast<BitsType>(x) + randomBits);
     return result;
   }
 

test/rand.cpp

diff --git a/test/rand.cpp b/test/rand.cpp
index b5cf801..6a7c316 100644
--- a/test/rand.cpp
+++ b/test/rand.cpp
@@ -75,27 +75,36 @@
   HistogramHelper(Scalar lower, Scalar upper, int nbins)
       : lower_{lower}, upper_{upper}, num_bins_{nbins}, bin_width_{bin_width(lower, upper, nbins)} {}
 
-  int bin(Scalar v) { return static_cast<int>(RangeType(v - lower_) / bin_width_); }
+  int bin(Scalar v) { return static_cast<int>(RangeType(RangeType(v) - RangeType(lower_)) / bin_width_); }
 
   double uniform_bin_probability(int bin) {
-    // Avoid overflow in computing range.
-    double range = static_cast<double>(RangeType(upper_ - lower_)) + 1.0;
+    // The full range upper - lower + 1 might overflow the RangeType by one.
+    // So instead, we know we have (nbins - 1) bins of width bin_width_,
+    // and the last bin of width:
+    RangeType last_bin_width =
+        RangeType(upper_) - (RangeType(lower_) + RangeType(num_bins_ - 1) * bin_width_) + RangeType(1);
+    double last_bin_ratio = static_cast<double>(last_bin_width) / static_cast<double>(bin_width_);
+    // Total probability = (nbins - 1) * p + last_bin_ratio * p = 1.0
+    // p = 1.0 / (nbins - 1 + last_bin_ratio)
+    double p = 1.0 / (last_bin_ratio + num_bins_ - 1);
     if (bin < num_bins_ - 1) {
-      return static_cast<double>(bin_width_) / range;
+      return p;
     }
-    return static_cast<double>(RangeType(upper_) - RangeType((lower_ + bin * bin_width_)) + 1) / range;
+    return last_bin_ratio * p;
   }
 
  private:
-  static constexpr Scalar bin_width(Scalar lower, Scalar upper, int nbins) {
+  static constexpr RangeType bin_width(Scalar lower, Scalar upper, int nbins) {
     // Avoid overflow in computing the full range.
-    return RangeType(upper - nbins - lower + 1) / nbins + 1;
+    // floor( (upper - lower + 1) / nbins) )
+    //    = floor( (upper- nbins - lower + 1 + nbins) / nbins) )
+    return RangeType(RangeType(upper - nbins) - RangeType(lower) + 1) / nbins + 1;
   }
 
   Scalar lower_;
   Scalar upper_;
   int num_bins_;
-  Scalar bin_width_;
+  RangeType bin_width_;
 };
 
 template <typename Scalar>