New rule proposal: Declare precedence when using null-coalescing operators

[b-jeltes-tjip]

If a null-coalescing operator is used in a more complex statement, precedence should be declared to guarantee that the order of operation matches the intention.

Example

Consider this code example: the developer wants to add the nullable integers a and b together and store the outcome in c. Using null-coalescing operators, the developer intends to substitute a and b independently with 0.

int? a = 1;
int? b = 3;

var c = a ?? 0 + b ?? 0;

Developer's expectation: c = 1 + 3
Actual outcome: c = 1

Developer's intended order of execution:

var c = (a ?? 0) + (b ?? 0);

The actual order of execution of the calculation in the example is as follows:

var c = a ?? (0 + (b ?? 0));

If a has a value, a is returned instead of the intended outcome of a + b.

Conclusion

A rule reminding the developer to declare precedence when using null-coalescing operators would be a good addition, as it would help prevent unintended outcomes.

Up for discussion

This could be a new rule, but it's also loosely related to SA1407ArithmeticExpressionsMustDeclarePrecedence so it could be desirable to add the null-coalescing operator to SA1407 as another operator considered in this rule.