Avoid infinite recursion for self-referencing generic type

Closes gh-32282
See gh-30079

Author: jhoeller

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

@@ -28,8 +28,10 @@
 import java.lang.reflect.WildcardType;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.HashSet;
 import java.util.IdentityHashMap;
 import java.util.Map;
+import java.util.Set;
 import java.util.StringJoiner;
 
 import org.springframework.core.SerializableTypeWrapper.FieldTypeProvider;
@@ -588,18 +590,28 @@ public boolean hasUnresolvableGenerics() {
 		if (this == NONE) {
 			return false;
 		}
+		return hasUnresolvableGenerics(null);
+	}
+
+	private boolean hasUnresolvableGenerics(@Nullable Set<Type> alreadySeen) {
 		Boolean unresolvableGenerics = this.unresolvableGenerics;
 		if (unresolvableGenerics == null) {
-			unresolvableGenerics = determineUnresolvableGenerics();
+			unresolvableGenerics = determineUnresolvableGenerics(alreadySeen);
 			this.unresolvableGenerics = unresolvableGenerics;
 		}
 		return unresolvableGenerics;
 	}
 
-	private boolean determineUnresolvableGenerics() {
+	private boolean determineUnresolvableGenerics(@Nullable Set<Type> alreadySeen) {
+		if (alreadySeen != null && alreadySeen.contains(this.type)) {
+			// Self-referencing generic -> not unresolvable
+			return false;
+		}
+
 		ResolvableType[] generics = getGenerics();
 		for (ResolvableType generic : generics) {
-			if (generic.isUnresolvableTypeVariable() || generic.isWildcardWithoutBounds() || generic.hasUnresolvableGenerics()) {
+			if (generic.isUnresolvableTypeVariable() || generic.isWildcardWithoutBounds() ||
+					generic.hasUnresolvableGenerics(currentTypeSeen(alreadySeen))) {
 				return true;
 			}
 		}
@@ -619,12 +631,20 @@ private boolean determineUnresolvableGenerics() {
 			}
 			Class<?> superclass = resolved.getSuperclass();
 			if (superclass != null && superclass != Object.class) {
-				return getSuperType().hasUnresolvableGenerics();
+				return getSuperType().hasUnresolvableGenerics(currentTypeSeen(alreadySeen));
 			}
 		}
 		return false;
 	}
 
+	private Set<Type> currentTypeSeen(@Nullable Set<Type> alreadySeen) {
+		if (alreadySeen == null) {
+			alreadySeen = new HashSet<>(4);
+		}
+		alreadySeen.add(this.type);
+		return alreadySeen;
+	}
+
 	/**
 	 * Determine whether the underlying type is a type variable that
 	 * cannot be resolved through the associated variable resolver.

spring-core/src/test/java/org/springframework/core/ResolvableTypeTests.java

@@ -1321,6 +1321,18 @@ void hasUnresolvableGenericsWhenNested() throws Exception {
 		assertThat(type.hasUnresolvableGenerics()).isTrue();
 	}
 
+	@Test
+	void hasUnresolvableGenericsWhenSelfReferring() {
+		ResolvableType type = ResolvableType.forInstance(new Bar());
+		assertThat(type.hasUnresolvableGenerics()).isFalse();
+	}
+
+	@Test
+	void hasUnresolvableGenericsWithEnum() {
+		ResolvableType type = ResolvableType.forType(SimpleEnum.class.getGenericSuperclass());
+		assertThat(type.hasUnresolvableGenerics()).isFalse();
+	}
+
 	@Test
 	void spr11219() throws Exception {
 		ResolvableType type = ResolvableType.forField(BaseProvider.class.getField("stuff"), BaseProvider.class);
@@ -1624,12 +1636,22 @@ interface ListOfGenericArray extends List<List<String>[]> {
 	}
 
 
-	public interface ListOfListSupplier<T> {
+	interface ListOfListSupplier<T> {
 
 		List<List<T>> get();
+	}
+
+
+	class Foo<T extends Foo<T>> {
+	}
 
+	class Bar extends Foo<Bar> {
 	}
 
+
+	enum SimpleEnum { VALUE }
+
+
 	static class EnclosedInParameterizedType<T> {
 
 		static class InnerRaw {