In reply to Shubhabrata:
It is not a conflict. Section 18.5.3 says
Absent any other constraints, the probability that the expression matches any value in the list is proportional to its specified weight. If there are constraints on some expressions that cause the distribution weights on these expressions to be not satisfiable, implementations are only required to satisfy the constraints. An exception to this rule is a weight of zero, which is treated as a constraint.
Note that the next revision of the LRM has clarified this paragraph to say
Distribution weights can be used to cause selected corner cases to occur more frequently than they would otherwise. However, nonzero distribution weights do not change the solution space and, therefore, cannot cause the solver to fail. If there are constraints that cause the distribution weights on some expressions to be unsatisfiable, implementations are only required to satisfy the constraints. The exceptions to this rule are values not specified by the distribution and values specified with a total weight of zero, both of which shall be treated as a constraint that the expression not have such values.