Resolve bounds for type variable before emptiness check

Closes gh-34328

Author: jhoeller

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

@@ -180,7 +180,7 @@ else if (genericType instanceof ParameterizedType parameterizedType) {
 								generics[i] = resolvedTypeArgument;
 							}
 							else {
-								generics[i] = ResolvableType.forType(typeArgument).resolveType();
+								generics[i] = ResolvableType.forType(typeArgument);
 							}
 						}
 						else if (typeArgument instanceof ParameterizedType) {
@@ -226,7 +226,7 @@ private static ResolvableType resolveVariable(TypeVariable<?> typeVariable, Reso
 				return resolvedType;
 			}
 		}
-		return ResolvableType.NONE;
+		return ResolvableType.forVariableBounds(typeVariable);
 	}
 
 	/**

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

@@ -953,14 +953,6 @@ ResolvableType resolveType() {
 		return NONE;
 	}
 
-	@Nullable
-	private Type resolveBounds(Type[] bounds) {
-		if (bounds.length == 0 || bounds[0] == Object.class) {
-			return null;
-		}
-		return bounds[0];
-	}
-
 	@Nullable
 	private ResolvableType resolveVariable(TypeVariable<?> variable) {
 		if (this.type instanceof TypeVariable) {
@@ -1463,6 +1455,24 @@ public static ResolvableType forArrayComponent(ResolvableType componentType) {
 		return new ResolvableType(arrayType, componentType, null, null);
 	}
 
+	/**
+	 * Return a {@code ResolvableType} for the bounds of the specified {@link TypeVariable}.
+	 * @param typeVariable the type variable
+	 * @return a {@code ResolvableType} for the specified bounds
+	 * @since 6.2.3
+	 */
+	static ResolvableType forVariableBounds(TypeVariable<?> typeVariable) {
+		return forType(resolveBounds(typeVariable.getBounds()));
+	}
+
+	@Nullable
+	private static Type resolveBounds(Type[] bounds) {
+		if (bounds.length == 0 || bounds[0] == Object.class) {
+			return null;
+		}
+		return bounds[0];
+	}
+
 	/**
 	 * Return a {@code ResolvableType} for the specified {@link Type}.
 	 * <p>Note: The resulting {@code ResolvableType} instance may not be {@link Serializable}.
@@ -1491,7 +1501,6 @@ public static ResolvableType forType(@Nullable Type type, @Nullable ResolvableTy
 		return forType(type, variableResolver);
 	}
 
-
 	/**
 	 * Return a {@code ResolvableType} for the specified {@link ParameterizedTypeReference}.
 	 * <p>Note: The resulting {@code ResolvableType} instance may not be {@link Serializable}.

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

@@ -212,19 +212,27 @@ void resolveNestedTypeVariable() throws Exception {
 	}
 
 	@Test
-	void resolvedTypeWithBase() {
-		Type type = method(WithBaseTypes.class, "get").getGenericReturnType();
-		Type resolvedType = resolveType(type, WithBaseTypes.class);
+	void resolveTypeWithElementBounds() {
+		Type type = method(WithElementBounds.class, "get").getGenericReturnType();
+		Type resolvedType = resolveType(type, WithElementBounds.class);
 		ParameterizedTypeReference<List<A>> reference = new ParameterizedTypeReference<>() {};
 		assertThat(resolvedType).isEqualTo(reference.getType());
 	}
 
+	@Test
+	void resolveTypeWithUnresolvableElement() {
+		Type type = method(WithUnresolvableElement.class, "get").getGenericReturnType();
+		Type resolvedType = resolveType(type, WithUnresolvableElement.class);
+		assertThat(resolvedType.toString()).isEqualTo("java.util.List<E>");
+	}
+
 	private static Method method(Class<?> target, String methodName, Class<?>... parameterTypes) {
 		Method method = findMethod(target, methodName, parameterTypes);
 		assertThat(method).describedAs(target.getName() + "#" + methodName).isNotNull();
 		return method;
 	}
 
+
 	public interface MyInterfaceType<T> {
 	}
 
@@ -348,33 +356,35 @@ public static class MySimpleTypeWithMethods extends MyTypeWithMethods<Integer> {
 	static class GenericClass<T> {
 	}
 
-	class A{}
+	class A {}
 
-	class B<T>{}
+	class B<T> {}
 
 	class C extends A {}
 
 	class D extends B<Long> {}
 
 	class E extends C {}
 
-	class TestIfc<T>{}
+	class TestIfc<T> {}
 
-	class TestImpl<I extends A, T extends B<I>> extends TestIfc<T>{
+	class TestImpl<I extends A, T extends B<I>> extends TestIfc<T> {
 	}
 
 	abstract static class BiGenericClass<T extends B<?>, V extends A> {}
 
-	abstract static class SpecializedBiGenericClass<U extends C> extends BiGenericClass<D, U>{}
+	abstract static class SpecializedBiGenericClass<U extends C> extends BiGenericClass<D, U> {}
 
 	static class TypeFixedBiGenericClass extends SpecializedBiGenericClass<E> {}
 
 	static class TopLevelClass<T> {
+
 		class Nested<X> {
 		}
 	}
 
 	static class TypedTopLevelClass extends TopLevelClass<Integer> {
+
 		class TypedNested extends Nested<Long> {
 		}
 	}
@@ -394,24 +404,31 @@ interface IdFixingRepository<T> extends Repository<T, Long> {
 	}
 
 	static class WithMethodParameter {
+
 		public void nestedGenerics(List<Map<String, Integer>> input) {
 		}
 	}
 
-	public interface ListOfListSupplier<T> {
+	interface ListOfListSupplier<T> {
 
 		List<List<T>> get();
 	}
 
-	public interface StringListOfListSupplier extends ListOfListSupplier<String> {
+	interface StringListOfListSupplier extends ListOfListSupplier<String> {
 	}
 
-	static class WithBaseTypes {
+	static class WithElementBounds {
 
 		<T extends A> List<T> get() {
 			return List.of();
 		}
+	}
+
+	static class WithUnresolvableElement<T> {
 
+		List<T> get() {
+			return List.of();
+		}
 	}
 
 }