Fix division bug discovered by https://svn.boost.org/trac/boost/ticke…

…t/11648

include/boost/multiprecision/cpp_int/divide.hpp

@@ -242,7 +242,21 @@ void divide_unsigned_helper(
       // Update r in a way that won't actually produce a negative result
       // in case the argument types are unsigned:
       //
-      if(r.compare(t) > 0)
+      if(truncated_t && carry)
+      {
+         // We need to calculate 2^n + t - r
+         // where n is the number of bits in this type.
+         // Simplest way is to get 2^n - r by complementing
+         // r, then add t to it.  Note that we can't call eval_complement
+         // in case this is a signed checked type:
+         for(unsigned i = 0; i <= r_order; ++i)
+            r.limbs()[i] = ~prem[i];
+         r.normalize();
+         eval_increment(r);
+         eval_add(r, t);
+         r_neg = !r_neg;
+      }
+      else if(r.compare(t) > 0)
       {
          eval_subtract(r, t);
       }

test/test_cpp_int.cpp

@@ -493,6 +493,16 @@ struct tester
       c = a ^ b;
       test_type d = ~(a ^ ~b);
       BOOST_CHECK_EQUAL(c, d);
+#endif
+#if defined(TEST2) || defined(TEST3)
+      // https://svn.boost.org/trac/boost/ticket/11648
+      a = (std::numeric_limits<test_type>::max)() - 69;
+      b = a / 139;
+      ++b;
+      c = a / b;
+      test_type r = a % b;
+      BOOST_CHECK(r < b);
+      BOOST_CHECK_EQUAL(a - c * b, r);
 #endif
    }