`IndexAccessor` cannot support custom `Collection` target type

[sbrannen]

Overview

Due to the current implementation of SpEL's Indexer, an IndexAccessor cannot provide custom support for a Collection target type.

The reason is that Indexer owns the Collection target type.

spring-framework/spring-expression/src/main/java/org/springframework/expression/spel/ast/Indexer.java

Lines 234 to 241 in a3afe51

	else if (target instanceof Collection<?> collection) {
	if (target instanceof List) {
	this.indexedType = IndexedType.LIST;
	}
	return new CollectionIndexingValueRef(collection, idx, targetDescriptor,
	state.getTypeConverter(), state.getConfiguration().isAutoGrowCollections(),
	state.getConfiguration().getMaximumAutoGrowSize());
	}

We should be able to ensure that a custom IndexAccessor can provide support for a Collection target type by separating our built-in List and Collection support and applying the built-in Collection support after custom index accessors have been applied, analogous to what we did in #32706. The built-in List support should still be applied before custom index accessors and therefore remain unchanged.

Related Issues

• Introduce IndexAccessor SPI to customize the SpEL Indexer #26478
• Custom IndexAccessor cannot support String index type #32706