Fix ResolvableType isAssignableFrom for <?>

Fix ResolvableType.isAssignableFrom to correctly deal with <?> style
generics.

Issue: SPR-10973

Author: Phillip Webb

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

@@ -138,7 +138,7 @@ private boolean isAssignableFrom(ResolvableType type, boolean checkingGeneric) {
 		Assert.notNull(type, "Type must not be null");
 
 		// If we cannot resolve types, we are not assignable
-		if (resolve() == null || type.resolve() == null) {
+		if (this == NONE || type == NONE) {
 			return false;
 		}
 
@@ -164,15 +164,16 @@ private boolean isAssignableFrom(ResolvableType type, boolean checkingGeneric) {
 		}
 
 		// Main assignability check
-		boolean rtn = resolve().isAssignableFrom(type.resolve());
+		boolean rtn = resolve(Object.class).isAssignableFrom(type.resolve(Object.class));
 
 		// We need an exact type match for generics
 		// List<CharSequence> is not assignable from List<String>
-		rtn &= (!checkingGeneric || resolve().equals(type.resolve()));
+		rtn &= (!checkingGeneric || resolve(Object.class).equals(type.resolve(Object.class)));
 
 		// Recursively check each generic
 		for (int i = 0; i < getGenerics().length; i++) {
-			rtn &= getGeneric(i).isAssignableFrom(type.as(resolve()).getGeneric(i), true);
+			rtn &= getGeneric(i).isAssignableFrom(
+					type.as(resolve(Object.class)).getGeneric(i), true);
 		}
 
 		return rtn;

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

@@ -52,8 +52,6 @@
 import static org.mockito.Matchers.*;
 import static org.mockito.Mockito.*;
 
-// FIXME nested
-
 /**
  * Tests for {@link ResolvableType}.
  *
@@ -822,8 +820,8 @@ public void isAssignableFromCannotBeResolved() throws Exception {
 		ResolvableType objectType = ResolvableType.forClass(Object.class);
 		ResolvableType unresolvableVariable = ResolvableType.forField(AssignmentBase.class.getField("o"));
 		assertThat(unresolvableVariable.resolve(), nullValue());
-		assertAssignable(objectType, unresolvableVariable).equalTo(false);
-		assertAssignable(unresolvableVariable, objectType).equalTo(false);
+		assertAssignable(objectType, unresolvableVariable).equalTo(true);
+		assertAssignable(unresolvableVariable, objectType).equalTo(true);
 	}
 
 	@Test
@@ -936,6 +934,7 @@ public void isAssignableFromForWildcards() throws Exception {
 		ResolvableType object = ResolvableType.forClass(Object.class);
 		ResolvableType charSequence = ResolvableType.forClass(CharSequence.class);
 		ResolvableType string = ResolvableType.forClass(String.class);
+		ResolvableType extendsAnon = ResolvableType.forField(AssignmentBase.class.getField("listAnon"), Assignment.class).getGeneric();
 		ResolvableType extendsObject = ResolvableType.forField(AssignmentBase.class.getField("listxo"), Assignment.class).getGeneric();
 		ResolvableType extendsCharSequence = ResolvableType.forField(AssignmentBase.class.getField("listxc"), Assignment.class).getGeneric();
 		ResolvableType extendsString = ResolvableType.forField(AssignmentBase.class.getField("listxs"), Assignment.class).getGeneric();
@@ -972,6 +971,8 @@ public void isAssignableFromForWildcards() throws Exception {
 				equalTo(false, true, true);
 		assertAssignable(charSequence, extendsObject, extendsCharSequence, extendsString).
 				equalTo(false, false, false);
+		assertAssignable(extendsAnon, object, charSequence, string).
+				equalTo(true, true, true);
 
 		// T <= ? super T
 		assertAssignable(superCharSequence, object, charSequence, string).
@@ -1144,6 +1145,8 @@ static class AssignmentBase<O, C, S> {
 
 		public List<S> lists;
 
+		public List<?> listAnon;
+
 		public List<? extends O> listxo;
 
 		public List<? extends C> listxc;