Support WildcardType resolution in GenericTypeResolver

This commit adds support for WildcardType bounds resolution,
commonly seen in Kotlin due to declaration-site variance,
but also possible in Java even if less common.

Closes gh-22313

Author: sdeleuze

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

@@ -39,6 +39,7 @@
  * @author Rob Harrop
  * @author Sam Brannen
  * @author Phillip Webb
+ * @author Sebastien Deleuze
  * @since 2.5.2
  */
 public final class GenericTypeResolver {
@@ -166,29 +167,30 @@ public static Type resolveType(Type genericType, @Nullable Class<?> contextClass
 			}
 			else if (genericType instanceof ParameterizedType parameterizedType) {
 				ResolvableType resolvedType = ResolvableType.forType(genericType);
-				if (resolvedType.hasUnresolvableGenerics()) {
-					Class<?>[] generics = new Class<?>[parameterizedType.getActualTypeArguments().length];
-					Type[] typeArguments = parameterizedType.getActualTypeArguments();
-					ResolvableType contextType = ResolvableType.forClass(contextClass);
-					for (int i = 0; i < typeArguments.length; i++) {
-						Type typeArgument = typeArguments[i];
-						if (typeArgument instanceof TypeVariable<?> typeVariable) {
-							ResolvableType resolvedTypeArgument = resolveVariable(typeVariable, contextType);
-							if (resolvedTypeArgument != ResolvableType.NONE) {
-								generics[i] = resolvedTypeArgument.resolve();
-							}
-							else {
-								generics[i] = ResolvableType.forType(typeArgument).resolve();
-							}
+				Class<?>[] generics = new Class<?>[parameterizedType.getActualTypeArguments().length];
+				Type[] typeArguments = parameterizedType.getActualTypeArguments();
+				ResolvableType contextType = ResolvableType.forClass(contextClass);
+				for (int i = 0; i < typeArguments.length; i++) {
+					Type typeArgument = typeArguments[i];
+					if (typeArgument instanceof TypeVariable<?> typeVariable) {
+						ResolvableType resolvedTypeArgument = resolveVariable(typeVariable, contextType);
+						if (resolvedTypeArgument != ResolvableType.NONE) {
+							generics[i] = resolvedTypeArgument.resolve();
 						}
 						else {
 							generics[i] = ResolvableType.forType(typeArgument).resolve();
 						}
 					}
-					Class<?> rawClass = resolvedType.getRawClass();
-					if (rawClass != null) {
-						return ResolvableType.forClassWithGenerics(rawClass, generics).getType();
+					else if (typeArgument instanceof WildcardType wildcardType) {
+						generics[i] = resolveWildcard(wildcardType, contextType).resolve();
 					}
+					else {
+						generics[i] = ResolvableType.forType(typeArgument).resolve();
+					}
+				}
+				Class<?> rawClass = resolvedType.getRawClass();
+				if (rawClass != null) {
+					return ResolvableType.forClassWithGenerics(rawClass, generics).getType();
 				}
 			}
 		}
@@ -224,6 +226,26 @@ private static ResolvableType resolveVariable(TypeVariable<?> typeVariable, Reso
 		return ResolvableType.NONE;
 	}
 
+	private static ResolvableType resolveWildcard(WildcardType wildcardType, ResolvableType contextType) {
+		for (Type bound : wildcardType.getUpperBounds()) {
+			if (bound instanceof TypeVariable<?> typeVariable) {
+				ResolvableType resolvedTypeArgument = resolveVariable(typeVariable, contextType);
+				if (resolvedTypeArgument != ResolvableType.NONE) {
+					return resolvedTypeArgument;
+				}
+			}
+		}
+		for (Type bound : wildcardType.getLowerBounds()) {
+			if (bound instanceof TypeVariable<?> typeVariable) {
+				ResolvableType resolvedTypeArgument = resolveVariable(typeVariable, contextType);
+				if (resolvedTypeArgument != ResolvableType.NONE) {
+					return resolvedTypeArgument;
+				}
+			}
+		}
+		return ResolvableType.forType(wildcardType);
+	}
+
 	/**
 	 * Resolve the specified generic type against the given TypeVariable map.
 	 * <p>Used by Spring Data.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,6 +37,7 @@
 /**
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Sebastien Deleuze
  */
 @SuppressWarnings({"unchecked", "rawtypes"})
 class GenericTypeResolverTests {
@@ -185,6 +186,51 @@ public void resolveTransitiveTypeVariableWithDifferentName() {
 		assertThat(resolved).isEqualTo(E.class);
 	}
 
+	@Test
+	void resolveWildcardTypeWithUpperBound() {
+		Method method = findMethod(MySimpleSuperclassType.class, "upperBound", List.class);
+		Type resolved = resolveType(method.getGenericParameterTypes()[0], MySimpleSuperclassType.class);
+		ResolvableType resolvableType = ResolvableType.forType(resolved);
+		assertThat(resolvableType.hasUnresolvableGenerics()).isFalse();
+		assertThat(resolvableType.resolveGenerics()).containsExactly(String.class);
+	}
+
+	@Test
+	void resolveWildcardTypeWithUpperBoundWithResolvedType() {
+		Method method = findMethod(MySimpleSuperclassType.class, "upperBoundWithResolvedType", List.class);
+		Type resolved = resolveType(method.getGenericParameterTypes()[0], MySimpleSuperclassType.class);
+		ResolvableType resolvableType = ResolvableType.forType(resolved);
+		assertThat(resolvableType.hasUnresolvableGenerics()).isFalse();
+		assertThat(resolvableType.resolveGenerics()).containsExactly(Integer.class);
+	}
+
+	@Test
+	void resolveWildcardTypeWithLowerBound() {
+		Method method = findMethod(MySimpleSuperclassType.class, "lowerBound", List.class);
+		Type resolved = resolveType(method.getGenericParameterTypes()[0], MySimpleSuperclassType.class);
+		ResolvableType resolvableType = ResolvableType.forType(resolved);
+		assertThat(resolvableType.hasUnresolvableGenerics()).isFalse();
+		assertThat(resolvableType.resolveGenerics()).containsExactly(String.class);
+	}
+
+	@Test
+	void resolveWildcardTypeWithLowerBoundWithResolvedType() {
+		Method method = findMethod(MySimpleSuperclassType.class, "lowerBoundWithResolvedType", List.class);
+		Type resolved = resolveType(method.getGenericParameterTypes()[0], MySimpleSuperclassType.class);
+		ResolvableType resolvableType = ResolvableType.forType(resolved);
+		assertThat(resolvableType.hasUnresolvableGenerics()).isFalse();
+		assertThat(resolvableType.resolveGenerics()).containsExactly(Integer.class);
+	}
+
+	@Test
+	void resolveWildcardTypeWithUnbounded() {
+		Method method = findMethod(MySimpleSuperclassType.class, "unbounded", List.class);
+		Type resolved = resolveType(method.getGenericParameterTypes()[0], MySimpleSuperclassType.class);
+		ResolvableType resolvableType = ResolvableType.forType(resolved);
+		assertThat(resolvableType.hasUnresolvableGenerics()).isFalse();
+		assertThat(resolvableType.resolveGenerics()).containsExactly(Object.class);
+	}
+
 	public interface MyInterfaceType<T> {
 	}
 
@@ -195,6 +241,21 @@ public class MyCollectionInterfaceType implements MyInterfaceType<Collection<Str
 	}
 
 	public abstract class MySuperclassType<T> {
+
+		public void upperBound(List<? extends T> list) {
+		}
+
+		public void upperBoundWithResolvedType(List<? extends Integer> list) {
+		}
+
+		public void lowerBound(List<? extends T> list) {
+		}
+
+		public void lowerBoundWithResolvedType(List<? super Integer> list) {
+		}
+
+		public void unbounded(List<?> list) {
+		}
 	}
 
 	public class MySimpleSuperclassType extends MySuperclassType<String> {