Avoid overflow further in rational::operator+=

include/boost/rational.hpp

@@ -516,6 +516,10 @@ inline rational<IntType> operator- (const rational<IntType>& r)
 template <typename IntType>
 BOOST_CXX14_CONSTEXPR rational<IntType>& rational<IntType>::operator+= (const rational<IntType>& r)
 {
+    // Prevent division by zero below if *this == 0, r == 0 and num_g == 0.
+    if (r.num == IntType(0))
+        return *this; // r == 0 => *this need not be changed.
+
     // This calculation avoids overflow, and minimises the number of expensive
     // calculations. Thanks to Nickolay Mladenov for this algorithm.
     //
@@ -534,15 +538,22 @@ BOOST_CXX14_CONSTEXPR rational<IntType>& rational<IntType>::operator+= (const ra
     // Which proves that instead of normalizing the result, it is better to
     // divide num and den by gcd((a*d1 + c*b1), g)
 
+    // First dividing, then multiplying by num_g avoids overflow even further,
+    // at the cost of a branch, an extra GCD calculation, two integer divisions
+    // and one integer multiplication.
+
     // Protect against self-modification
     IntType r_num = r.num;
     IntType r_den = r.den;
 
+    const IntType num_g = integer::gcd(num, r_num);
     IntType g = integer::gcd(den, r_den);
     den /= g;  // = b1 from the calculations above
-    num = num * (r_den / g) + r_num * den;
+    num = (num / num_g) * (r_den / g) + (r_num / num_g) * den;
+
     g = integer::gcd(num, g);
     num /= g;
+    num *= num_g;
     den *= r_den/g;
 
     return *this;

test/rational_test.cpp

@@ -1478,6 +1478,39 @@ BOOST_AUTO_TEST_SUITE_END()
 // The bugs, patches, and requests checking suite
 BOOST_AUTO_TEST_SUITE( bug_patch_request_suite )
 
+BOOST_AUTO_TEST_CASE( operator_plus_overflow_test )
+{
+    MyOverflowingUnsigned const base = 10;
+    MyOverflowingUnsigned num1 = 1;
+    while ( num1 < UINT_MAX / base ) num1 *= base;
+    if ( UINT_MAX / num1 < 4 ) // e.g. 64-bit unsigned
+    {
+        num1 /= base;
+        num1 *= 7;
+    }
+    else // e.g. 32-bit unsigned
+    {
+        num1 *= 2;
+    }
+    MyOverflowingUnsigned const num2 = num1 * 2;
+
+    // Other unsigned types are not covered by this test
+    // (but it should still pass on all platforms).
+    if ( sizeof( MyOverflowingUnsigned ) == 4
+      || sizeof( MyOverflowingUnsigned ) == 8 )
+    {
+        BOOST_TEST( UINT_MAX - num1 < num2 );
+    }
+
+    MyOverflowingUnsigned const denom = 3;
+    boost::rational<MyOverflowingUnsigned> const r1( num1, denom ),
+                                                 r2( num2, denom );
+    // This test succeeds when MyOverflowingUnsigned is replaced with a built-in
+    // unsigned type due to unsigned integer overflow wraparound. But it guards
+    // against the undefined behavior of signed integer overflow.
+    BOOST_TEST( r1 + r2 == num1 );
+}
+
 // "rational operator< can overflow"
 BOOST_AUTO_TEST_CASE( bug_798357_test )
 {