Restore binding of system environment using legacy names

Remove `Binder` optimization introduced in commit 8f14dca164 so that
legacy format names in environment variables can be used again.

Fixes gh-46184

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/IndexedElementsBinder.java

@@ -107,7 +107,6 @@ abstract class IndexedElementsBinder<T> extends AggregateBinder<T> {
 			AggregateElementBinder elementBinder, IndexedCollectionSupplier collection, ResolvableType elementType) {
 		Set<String> knownIndexedChildren = Collections.emptySet();
 		if (source instanceof IterableConfigurationPropertySource iterableSource) {
-			source = iterableSource.filter(root::isAncestorOf);
 			knownIndexedChildren = getKnownIndexedChildren(iterableSource, root);
 		}
 		for (int i = 0; i < Integer.MAX_VALUE; i++) {
@@ -124,10 +123,10 @@ abstract class IndexedElementsBinder<T> extends AggregateBinder<T> {
 		}
 	}
 
-	private Set<String> getKnownIndexedChildren(IterableConfigurationPropertySource filteredSource,
+	private Set<String> getKnownIndexedChildren(IterableConfigurationPropertySource source,
 			ConfigurationPropertyName root) {
 		Set<String> knownIndexedChildren = new HashSet<>();
-		for (ConfigurationPropertyName name : filteredSource) {
+		for (ConfigurationPropertyName name : source.filter(root::isAncestorOf)) {
 			ConfigurationPropertyName choppedName = name.chop(root.getNumberOfElements() + 1);
 			if (choppedName.isLastElementIndexed()) {
 				knownIndexedChildren.add(choppedName.getLastElement(Form.UNIFORM));
@@ -136,17 +135,17 @@ abstract class IndexedElementsBinder<T> extends AggregateBinder<T> {
 		return knownIndexedChildren;
 	}
 
-	private void assertNoUnboundChildren(Set<String> unboundIndexedChildren,
-			IterableConfigurationPropertySource filteredSource, ConfigurationPropertyName root) {
+	private void assertNoUnboundChildren(Set<String> unboundIndexedChildren, IterableConfigurationPropertySource source,
+			ConfigurationPropertyName root) {
 		if (unboundIndexedChildren.isEmpty()) {
 			return;
 		}
 		Set<ConfigurationProperty> unboundProperties = new TreeSet<>();
-		for (ConfigurationPropertyName name : filteredSource) {
+		for (ConfigurationPropertyName name : source.filter(root::isAncestorOf)) {
 			ConfigurationPropertyName choppedName = name.chop(root.getNumberOfElements() + 1);
 			if (choppedName.isLastElementIndexed()
 					&& unboundIndexedChildren.contains(choppedName.getLastElement(Form.UNIFORM))) {
-				unboundProperties.add(filteredSource.getConfigurationProperty(name));
+				unboundProperties.add(source.getConfigurationProperty(name));
 			}
 		}
 		if (!unboundProperties.isEmpty()) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/CollectionBinderTests.java

@@ -19,9 +19,11 @@ package org.springframework.boot.context.properties.bind;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.EnumSet;
+import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import org.junit.jupiter.api.Test;
@@ -33,6 +35,7 @@ import org.springframework.boot.context.properties.source.ConfigurationPropertyS
 import org.springframework.boot.context.properties.source.MockConfigurationPropertySource;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.env.StandardEnvironment;
+import org.springframework.core.env.SystemEnvironmentPropertySource;
 import org.springframework.test.context.support.TestPropertySourceUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -465,6 +468,36 @@ class CollectionBinderTests {
 		assertThat(result.getValues().get(0)).containsExactly(ExampleEnum.FOO_BAR, ExampleEnum.BAR_BAZ);
 	}
 
+	@Test
+	void bindToWellFormedSystemEnvironmentVariableProperty() {
+		// gh-46184
+		Map<String, Object> map = new LinkedHashMap<>();
+		map.put("FOO_THENAMES_0_FIRST", "spring");
+		map.put("FOO_THENAMES_0_LAST", "boot");
+		map.put("FOO_THENAMES_1_FIRST", "binding");
+		map.put("FOO_THENAMES_1_LAST", "test");
+		SystemEnvironmentPropertySource propertySource = new SystemEnvironmentPropertySource(
+				StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, map);
+		this.sources.add(ConfigurationPropertySource.from(propertySource));
+		BeanWithCamelCaseNameList result = this.binder.bind("foo", BeanWithCamelCaseNameList.class).get();
+		assertThat(result.theNames()).containsExactly(new Name("spring", "boot"), new Name("binding", "test"));
+	}
+
+	@Test
+	void bindToLegacySystemEnvironmentVariableProperty() {
+		// gh-46184
+		Map<String, Object> map = new LinkedHashMap<>();
+		map.put("FOO_THE_NAMES_0_FIRST", "spring");
+		map.put("FOO_THE_NAMES_0_LAST", "boot");
+		map.put("FOO_THE_NAMES_1_FIRST", "binding");
+		map.put("FOO_THE_NAMES_1_LAST", "test");
+		SystemEnvironmentPropertySource propertySource = new SystemEnvironmentPropertySource(
+				StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, map);
+		this.sources.add(ConfigurationPropertySource.from(propertySource));
+		BeanWithCamelCaseNameList result = this.binder.bind("foo", BeanWithCamelCaseNameList.class).get();
+		assertThat(result.theNames()).containsExactly(new Name("spring", "boot"), new Name("binding", "test"));
+	}
+
 	static class ExampleCollectionBean {
 
 		private final List<String> items = new ArrayList<>();
@@ -607,6 +640,10 @@ class CollectionBinderTests {
 
 	}
 
+	record BeanWithCamelCaseNameList(List<Name> theNames) {
+
+	}
+
 	record Name(String first, String last) {
 
 	}