Support binding to collection with EnumSet values

Fixes gh-15539

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

@@ -46,7 +46,8 @@ class CollectionBinder extends IndexedElementsBinder<Collection<Object>> {
 				target.getType().asCollection().getGenerics());
 		ResolvableType elementType = target.getType().asCollection().getGeneric();
 		IndexedCollectionSupplier result = new IndexedCollectionSupplier(
-				() -> CollectionFactory.createCollection(collectionType, 0));
+				() -> CollectionFactory.createCollection(collectionType,
+						elementType.resolve(), 0));
 		bindIndexed(name, target, elementBinder, aggregateType, elementType, result);
 		if (result.wasSupplied()) {
 			return result.get();

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

@@ -18,6 +18,7 @@ package org.springframework.boot.context.properties.bind;
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
@@ -27,6 +28,7 @@ import java.util.stream.Collectors;
 import org.junit.Before;
 import org.junit.Test;
 
+import org.springframework.boot.context.properties.bind.BinderTests.ExampleEnum;
 import org.springframework.boot.context.properties.bind.BinderTests.JavaBean;
 import org.springframework.boot.context.properties.source.ConfigurationProperty;
 import org.springframework.boot.context.properties.source.ConfigurationPropertySource;
@@ -448,6 +450,17 @@ public class CollectionBinderTests {
 		assertThat(result.getValues()).containsExactly("a", "b", "c");
 	}
 
+	@Test
+	public void bindToBeanWithEnumSetCollection() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.values[0]", "foo-bar,bar-baz");
+		this.sources.add(source);
+		BeanWithEnumsetCollection result = this.binder
+				.bind("foo", Bindable.of(BeanWithEnumsetCollection.class)).get();
+		assertThat(result.getValues().get(0)).containsExactly(ExampleEnum.FOO_BAR,
+				ExampleEnum.BAR_BAZ);
+	}
+
 	public static class ExampleCollectionBean {
 
 		private List<String> items = new ArrayList<>();
@@ -566,4 +579,18 @@ public class CollectionBinderTests {
 
 	}
 
+	public static class BeanWithEnumsetCollection {
+
+		private List<EnumSet<ExampleEnum>> values;
+
+		public void setValues(List<EnumSet<ExampleEnum>> values) {
+			this.values = values;
+		}
+
+		public List<EnumSet<ExampleEnum>> getValues() {
+			return this.values;
+		}
+
+	}
+
 }