Support Collection target types in custom IndexAccessors

Prior to this commit, an IndexAccessor could not provide support for a
Collection target type, since the built-in support for indexing into a
Collection in SpEL's Indexer took precedence.

This commit allows an IndexAccessor to support custom Collection target
types by separating the built-in List and Collection support and
applying the built-in Collection support after custom index accessors
have been applied.

Closes gh-32736

spring-expression/src/test/java/org/springframework/expression/spel/IndexingTests.java

@@ -21,9 +21,12 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 import java.math.BigDecimal;
+import java.util.AbstractCollection;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.HashMap;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.NoSuchElementException;
@@ -44,6 +47,7 @@ import org.springframework.expression.IndexAccessor;
 import org.springframework.expression.PropertyAccessor;
 import org.springframework.expression.TypedValue;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.expression.spel.support.ReflectiveIndexAccessor;
 import org.springframework.expression.spel.support.SimpleEvaluationContext;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
 import org.springframework.expression.spel.testresources.Person;
@@ -735,6 +739,25 @@ class IndexingTests {
 					.havingCause().withMessage("unknown bird: property");
 		}
 
+		@Test  // gh-32736
+		void readIndexWithCollectionTargetType() {
+			context.addIndexAccessor(new ColorCollectionIndexAccessor());
+
+			Expression expression = parser.parseExpression("[0]");
+
+			// List.of() relies on built-in list support.
+			assertThat(expression.getValue(context, List.of(Color.RED))).isEqualTo(Color.RED);
+
+			ColorCollection colorCollection = new ColorCollection();
+
+			// Preconditions for this use case.
+			assertThat(colorCollection).isInstanceOf(Collection.class);
+			assertThat(colorCollection).isNotInstanceOf(List.class);
+
+			// ColorCollection relies on custom ColorCollectionIndexAccessor.
+			assertThat(expression.getValue(context, colorCollection)).isEqualTo(Color.RED);
+		}
+
 		static class BirdNameToColorMappings {
 
 			public final String property = "enigma";
@@ -755,6 +778,34 @@ class IndexingTests {
 			}
 		}
 
+		static class ColorCollection extends AbstractCollection<Color> {
+
+			public Color get(int index) {
+				return switch (index) {
+					case 0 -> Color.RED;
+					case 1 -> Color.BLUE;
+					default -> throw new NoSuchElementException("No color at index " + index);
+				};
+			}
+
+			@Override
+			public Iterator<Color> iterator() {
+				throw new UnsupportedOperationException();
+			}
+
+			@Override
+			public int size() {
+				throw new UnsupportedOperationException();
+			}
+		}
+
+		static class ColorCollectionIndexAccessor extends ReflectiveIndexAccessor {
+
+			ColorCollectionIndexAccessor() {
+				super(ColorCollection.class, int.class, "get");
+			}
+		}
+
 		/**
 		 * {@link IndexAccessor} that knows how to read and write indexes in a
 		 * Jackson {@link ArrayNode}.