Restore previous source in Context.withSource calls

Fix `Context.withSource` is the `Binder` to ensure that the previous
source is restored once the callback has completed.

This update is especially important in Spring Boot 3.5+ since indexed
binding calls `withSource` multiple times.

Fixes gh-46039

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -567,6 +567,7 @@ public class Binder {
 			if (source == null) {
 				return supplier.get();
 			}
+			ConfigurationPropertySource previous = this.source.get(0);
 			this.source.set(0, source);
 			this.sourcePushCount++;
 			try {
@@ -574,6 +575,7 @@ public class Binder {
 			}
 			finally {
 				this.sourcePushCount--;
+				this.source.set(0, previous);
 			}
 		}
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -123,6 +123,41 @@ class CollectionBinderTests {
 			});
 	}
 
+	@Test
+	void bindToCollectionWhenNonKnownIndexedChildNotBoundThrowsException() {
+		// gh-45994
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo[0].first", "Spring");
+		source.put("foo[0].last", "Boot");
+		source.put("foo[1].missing", "bad");
+		this.sources.add(source);
+		assertThatExceptionOfType(BindException.class)
+			.isThrownBy(() -> this.binder.bind("foo", Bindable.listOf(Name.class)))
+			.satisfies((ex) -> {
+				Set<ConfigurationProperty> unbound = ((UnboundConfigurationPropertiesException) ex.getCause())
+					.getUnboundProperties();
+				assertThat(unbound).hasSize(1);
+				ConfigurationProperty property = unbound.iterator().next();
+				assertThat(property.getName()).hasToString("foo[1].missing");
+				assertThat(property.getValue()).isEqualTo("bad");
+			});
+	}
+
+	@Test
+	void bindToNestedCollectionWhenNonKnownIndexed() {
+		// gh-46039
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo[0].items[0]", "a");
+		source.put("foo[0].items[1]", "b");
+		source.put("foo[0].string", "test");
+		this.sources.add(source);
+		List<ExampleCollectionBean> list = this.binder.bind("foo", Bindable.listOf(ExampleCollectionBean.class)).get();
+		assertThat(list).hasSize(1);
+		ExampleCollectionBean bean = list.get(0);
+		assertThat(bean.getItems()).containsExactly("a", "b", "d");
+		assertThat(bean.getString()).isEqualTo("test");
+	}
+
 	@Test
 	void bindToNonScalarCollectionWhenNonSequentialShouldThrowException() {
 		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
@@ -436,6 +471,8 @@ class CollectionBinderTests {
 
 		private Set<String> itemsSet = new LinkedHashSet<>();
 
+		private String string;
+
 		List<String> getItems() {
 			return this.items;
 		}
@@ -452,6 +489,14 @@ class CollectionBinderTests {
 			this.itemsSet = itemsSet;
 		}
 
+		String getString() {
+			return this.string;
+		}
+
+		void setString(String string) {
+			this.string = string;
+		}
+
 	}
 
 	static class ExampleCustomNoDefaultConstructorBean {
@@ -562,4 +607,8 @@ class CollectionBinderTests {
 
 	}
 
+	record Name(String first, String last) {
+
+	}
+
 }