This is another waited feature for the syntax of the expression-language component. The null-coalescing operator ?? becomes a need for variant programming needs these days.

Following my previous PR introducing the null-safe operator (#45795). I'm hereby introducing yet another essential operator to make the syntax even more complete.

The null-coalescing operator is a syntactic sugar for a common use of ternary in conjunction with isset() (in PHP) or equivalent in other languages. This is such a common use-case to the point that almost all majors programming syntax nowadays support a sort of a short-hand for that operation namely coalescing operator. Now it's time for the syntax of Expression-Language to do so!

Expressions like:

• foo.bar ?? 'default'
• foo[3] ?? 'default'
• foo.bar ?? foo['bar'] ?? 'default'

will default to the expression in the right-hand-side of the ?? operator whenever the expression in the left-hand-side of it does not exist or it's null. Note that this coalescing behavior can be chained and the validation logic takes decreasing priority from left to right.