Make nullability flexible in ConverterFactory#getConverter

This commits udpates ConverterFactory#getConverter to accept both
nullable and non-null T in the Converter return value.

Flexible nullability at ConverterFactory type level would have been
ideal, but is not possible due to how Kotlin deals with Class<T> when
T is nullable.

See gh-36063

spring-core/src/main/java/org/springframework/core/convert/converter/ConverterFactory.java

@@ -38,6 +38,6 @@ public interface ConverterFactory<S, R> {
 	 * @param targetType the target type to convert to
 	 * @return a converter from S to T
 	 */
-	<T extends R> Converter<S, @Nullable T> getConverter(Class<T> targetType);
+	<T extends R> Converter<S, ? extends @Nullable T> getConverter(Class<T> targetType);
 
 }

spring-core/src/test/kotlin/org/springframework/core/convert/converter/ConverterFactoryNullnessTests.kt

@@ -22,18 +22,27 @@ import kotlin.reflect.full.primaryConstructor
 
 /**
  * @author Brian Clozel
+ * @author Sebastien Deleuze
  */
 class ConverterFactoryNullnessTests {
 
 	@Test
 	fun converterFactoryWithNullableTypes() {
+		val factory = StringToNullableIdConverterFactory
+
+		val userIdConverter: Converter<String, UserId?> = factory.getConverter(UserId::class.java)
+		assertThat(userIdConverter.convert("42")).isEqualTo(UserId("42"))
+	}
+
+	@Test
+	fun converterFactoryWithNonNullableTypes() {
 		val factory = StringToIdConverterFactory
 
-		val userIdConverter = factory.getConverter(UserId::class.java)
+		val userIdConverter: Converter<String, UserId> = factory.getConverter(UserId::class.java)
 		assertThat(userIdConverter.convert("42")).isEqualTo(UserId("42"))
 	}
 
-	object StringToIdConverterFactory : ConverterFactory<String, Id> {
+	object StringToNullableIdConverterFactory : ConverterFactory<String, Id> {
 		override fun <T : Id> getConverter(targetType: Class<T>): Converter<String, T?> {
 			val constructor = checkNotNull(targetType.kotlin.primaryConstructor)
 			return Converter { source ->
@@ -42,12 +51,19 @@ class ConverterFactoryNullnessTests {
 		}
 	}
 
+	object StringToIdConverterFactory : ConverterFactory<String, Id> {
+		override fun <T : Id> getConverter(targetType: Class<T>): Converter<String, T> {
+			val constructor = checkNotNull(targetType.kotlin.primaryConstructor)
+			return Converter { source ->
+				constructor.call(source)
+			}
+		}
+	}
+
 	abstract class Id {
 		abstract val value: String
 	}
 
 	data class UserId(override val value: String) : Id()
 
-	data class ProductId(override val value: String) : Id()
-
 }