Overview

MethodReference and PropertyOrFieldReference already define local isNullSafe() methods, but we now need identical methods in Selection, Projection, and Indexer, and we may potentially need null-safe support for additional operators in the future.

However, doing so without a common mechanism will result in an if-block that's required to check each concrete node type separately.

In other words, the current if-block:

https://github.com/spring-projects/spring-framework/blob/b1b9ee0ccf1d6eaaea9c8ba15aec1bc110143e20/spring-expression/src/main/java/org/springframework/expression/spel/ast/CompoundExpression.java#L127-L128

... would grow to something similar to the following:

if ((nextChild instanceof MethodReference methodRef && methodRef.isNullSafe()) ||
        (nextChild instanceof PropertyOrFieldReference pofRef && pofRef.isNullSafe()) ||
        (nextChild instanceof Selection selection && selection.isNullSafe()) ||
        (nextChild instanceof Projection projection && projection.isNullSafe()) ||
        (nextChild instanceof Indexer indexer && indexer.isNullSafe())) {

Whereas, something unified like the following would be preferred.

if (nextChild.isNullSafe()) {

Prerequisite For

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