Fix ParameterizedTypeReference nullness

This commit ensures that `ParameterizedTypeReference<T>` can accept
nullable types. This is especially useful for Kotlin extension functions
and assertion contracts.

Fixes gh-36477

spring-core/src/main/java/org/springframework/core/ParameterizedTypeReference.java

@@ -42,7 +42,7 @@ import org.springframework.util.Assert;
  * @param <T> the referenced type
  * @see <a href="https://gafter.blogspot.nl/2006/12/super-type-tokens.html">Neal Gafter on Super Type Tokens</a>
  */
-public abstract class ParameterizedTypeReference<T> {
+public abstract class ParameterizedTypeReference<T extends @Nullable Object> {
 
 	private final Type type;
 

spring-messaging/src/test/kotlin/org/springframework/messaging/rsocket/RSocketRequesterExtensionsTests.kt

@@ -107,35 +107,35 @@ class RSocketRequesterExtensionsTests {
 	suspend fun retrieveAndAwait() {
 		val response = "foo"
 		val retrieveSpec = mockk<RSocketRequester.RetrieveSpec>()
-		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Mono.just("foo")
+		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<String>>(stringTypeRefMatcher)) } returns Mono.just("foo")
 		assertThat(retrieveSpec.retrieveAndAwait<String>()).isEqualTo(response)
 	}
 
 	@Test
 	suspend fun retrieveAndAwaitOrNull() {
 		val retrieveSpec = mockk<RSocketRequester.RetrieveSpec>()
-		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Mono.empty()
+		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<String>>(stringTypeRefMatcher)) } returns Mono.empty()
 		assertThat(retrieveSpec.retrieveAndAwaitOrNull<String>()).isNull()
 	}
 
 	@Test
 	suspend fun retrieveFlow() {
 		val retrieveSpec = mockk<RSocketRequester.RetrieveSpec>()
-		every { retrieveSpec.retrieveFlux(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Flux.just("foo", "bar")
+		every { retrieveSpec.retrieveFlux(match<ParameterizedTypeReference<String>>(stringTypeRefMatcher)) } returns Flux.just("foo", "bar")
 		assertThat(retrieveSpec.retrieveFlow<String>().toList()).contains("foo", "bar")
 	}
 
 	@Test
 	fun retrieveMono() {
 		val retrieveSpec = mockk<RSocketRequester.RetrieveSpec>()
-		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Mono.just("foo")
+		every { retrieveSpec.retrieveMono(match<ParameterizedTypeReference<String>>(stringTypeRefMatcher)) } returns Mono.just("foo")
 		assertThat(retrieveSpec.retrieveMono<String>().block()).isEqualTo("foo")
 	}
 
 	@Test
 	fun retrieveFlux() {
 		val retrieveSpec = mockk<RSocketRequester.RetrieveSpec>()
-		every { retrieveSpec.retrieveFlux(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Flux.just("foo", "bar")
+		every { retrieveSpec.retrieveFlux(match<ParameterizedTypeReference<String>>(stringTypeRefMatcher)) } returns Flux.just("foo", "bar")
 		assertThat(retrieveSpec.retrieveFlux<String>().collectList().block()).contains("foo", "bar")
 	}
 }