Only use Map converter binding when no descendants

Update `MapBinder` to only attempt conversion based binding when there
are no know descendant elements.

See gh-11892

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

@@ -24,6 +24,7 @@ import org.springframework.boot.context.properties.source.ConfigurationProperty;
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName.Form;
 import org.springframework.boot.context.properties.source.ConfigurationPropertySource;
+import org.springframework.boot.context.properties.source.ConfigurationPropertyState;
 import org.springframework.boot.context.properties.source.IterableConfigurationPropertySource;
 import org.springframework.core.CollectionFactory;
 import org.springframework.core.ResolvableType;
@@ -55,10 +56,12 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 		Map<Object, Object> map = CollectionFactory.createMap(
 				(target.getValue() == null ? target.getType().resolve() : Map.class), 0);
 		Bindable<?> resolvedTarget = resolveTarget(target);
+		boolean hasDescendants = getContext().streamSources().anyMatch((source) -> source
+				.containsDescendantOf(name) == ConfigurationPropertyState.PRESENT);
 		for (ConfigurationPropertySource source : getContext().getSources()) {
 			if (!ConfigurationPropertyName.EMPTY.equals(name)) {
 				ConfigurationProperty property = source.getConfigurationProperty(name);
-				if (property != null) {
+				if (property != null && !hasDescendants) {
 					Object value = getContext().getPlaceholdersResolver()
 							.resolvePlaceholders(property.getValue());
 					return ResolvableTypeDescriptor.forType(target.getType())

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

@@ -545,6 +545,22 @@ public class MapBinderTests {
 		assertThat(map.get("b")).isNotNull();
 	}
 
+	@Test
+	public void bindToMapWithCustomConverterAndChildElements() {
+		// gh-11892
+		DefaultConversionService conversionService = new DefaultConversionService();
+		conversionService.addConverter(new MapConverter());
+		Binder binder = new Binder(this.sources, null, conversionService);
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo", "boom");
+		source.put("foo.a", "a");
+		source.put("foo.b", "b");
+		this.sources.add(source);
+		Map<String, String> map = binder.bind("foo", STRING_STRING_MAP).get();
+		assertThat(map.get("a")).isEqualTo("a");
+		assertThat(map.get("b")).isEqualTo("b");
+	}
+
 	@Test
 	public void bindToMapWithNoConverterForValue() {
 		MockConfigurationPropertySource source = new MockConfigurationPropertySource();