igamma_inv query on double_fp_backend

Author: ckormanyos

include/boost/math/special_functions/detail/igamma_inverse.hpp

@@ -390,7 +390,9 @@ struct gamma_p_inverse_func
          f2 = -f2;
       }
 
-      return boost::math::make_tuple(static_cast<T>(f - p), f1, f2);
+      const T rela_diff_or_diff { (fabs(1 - p / f) <= 4 * tools::epsilon<T>()) ? T { 0 } : static_cast<T>(f - p) };
+
+      return boost::math::make_tuple(rela_diff_or_diff, f1, f2);
    }
 private:
    T a, p;