Relax List input check in GraphQlArgumentBinder

Allow any List type, not only ArrayList and LinkedList but also others
like SingletonList.

Fixes gh-410

spring-graphql/src/main/java/org/springframework/graphql/data/GraphQlArgumentBinder.java

@@ -128,7 +128,7 @@ public class GraphQlArgumentBinder {
 		try {
 			// From Collection
 
-			if (CollectionFactory.isApproximableCollectionType(rawValue.getClass())) {
+			if (isApproximableCollectionType(rawValue)) {
 				segments.push(argumentName);
 				return createCollection((Collection<Object>) rawValue, targetType, bindingResult, segments);
 			}
@@ -164,6 +164,11 @@ public class GraphQlArgumentBinder {
 		return (type.resolve(Object.class).equals(Optional.class) ? Optional.ofNullable(value) : value);
 	}
 
+	private boolean isApproximableCollectionType(Object rawValue) {
+		return (CollectionFactory.isApproximableCollectionType(rawValue.getClass()) ||
+				rawValue instanceof List);  // it may be SingletonList
+	}
+
 	@SuppressWarnings({"ConstantConditions", "unchecked"})
 	private <T> Collection<T> createCollection(
 			Collection<Object> rawCollection, ResolvableType collectionType,
@@ -253,7 +258,7 @@ public class GraphQlArgumentBinder {
 			if (rawValue == null && methodParam.isOptional()) {
 				args[i] = (paramTypes[i] == Optional.class ? Optional.empty() : null);
 			}
-			else if (rawValue != null && CollectionFactory.isApproximableCollectionType(rawValue.getClass())) {
+			else if (rawValue != null && isApproximableCollectionType(rawValue)) {
 				ResolvableType elementType = ResolvableType.forMethodParameter(methodParam);
 				args[i] = createCollection((Collection<Object>) rawValue, elementType, bindingResult, segments);
 			}

spring-graphql/src/test/java/org/springframework/graphql/data/GraphQlArgumentBinderTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.graphql.data;
 
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
@@ -224,6 +226,31 @@ class GraphQlArgumentBinderTests {
 				});
 	}
 
+	@Test // gh-410
+	void coercionWithSingletonList() throws Exception {
+
+		Map<String, String> itemMap = new HashMap<>();
+		itemMap.put("name", "Joe");
+		itemMap.put("age", "37");
+
+		Map<String, Object> arguments = new HashMap<>();
+		arguments.put("key", Collections.singletonList(itemMap));
+
+		DataFetchingEnvironment environment =
+				DataFetchingEnvironmentImpl.newDataFetchingEnvironment().arguments(arguments).build();
+
+		Object result = this.binder.bind(environment, "key",
+				ResolvableType.forClassWithGenerics(List.class, Item.class));
+
+		assertThat(result).isNotNull().isInstanceOf(List.class);
+		List<Item> items = (List<Item>) result;
+
+		assertThat(items).hasSize(1);
+		assertThat(items.get(0).getName()).isEqualTo("Joe");
+		assertThat(items.get(0).getAge()).isEqualTo(37);
+	}
+
+
 	@SuppressWarnings("unchecked")
 	private DataFetchingEnvironment environment(String jsonPayload) throws JsonProcessingException {
 		Map<String, Object> arguments = this.mapper.readValue(jsonPayload, Map.class);