Fix regression with covariant property return types

Prior to this change, the Property class introduced in 3.1 M2 validated
read/write property method pairs based on whether their parameter/return
types were equal to one another.  This precluded the valid possibility
of read method that returns a subtype of the write method's parameter
type, and represented a regression against 3.1 M1 and earlier versions.

The implementation now uses isAssignableFrom rather than a straight
equals check against the types.

Issue: SPR-8432

org.springframework.core/src/main/java/org/springframework/core/convert/Property.java

@@ -133,8 +133,8 @@ public final class Property {
 		if (read == null && write == null) {
 			throw new IllegalStateException("Property is neither readable nor writeable");
 		}
-		if (read != null && write != null && !read.getParameterType().equals(write.getParameterType())) {
+		if (read != null && write != null && !write.getParameterType().isAssignableFrom(read.getParameterType())) {
-			throw new IllegalStateException("Read and write parameter types are not the same");
+			throw new IllegalStateException("Write parameter is not assignable from read parameter");
 		}
 		return read != null ? read : write;
 	}

org.springframework.core/src/test/java/org/springframework/core/convert/TypeDescriptorTests.java

@@ -226,6 +226,14 @@ public class TypeDescriptorTests {
 		assertEquals(Integer.class, desc.getType());
 	}
 
+	@Test
+	public void propertyTypeCovariance() throws Exception {
+		GenericType<Number> genericBean = new NumberType();
+		Property property = new Property(getClass(), genericBean.getClass().getMethod("getProperty", null), genericBean.getClass().getMethod("setProperty", Number.class));		
+		TypeDescriptor desc = new TypeDescriptor(property);
+		assertEquals(Integer.class, desc.getType());
+	}
+
 	@Test
 	public void propertyGenericTypeList() throws Exception {
 		GenericType<Integer> genericBean = new IntegerType();
@@ -239,7 +247,7 @@ public class TypeDescriptorTests {
 		T getProperty();
 	
 		void setProperty(T t);
-		
+
 		List<T> getListProperty();
 		
 		void setListProperty(List<T> t);
@@ -270,6 +278,23 @@ public class TypeDescriptorTests {
 		
 	}
 
+	public class NumberType implements GenericType<Number> {
+
+		public Integer getProperty() {
+			return null;
+		}
+
+		public void setProperty(Number t) {
+		}
+
+		public List<Number> getListProperty() {
+			return null;
+		}
+
+		public void setListProperty(List<Number> t) {
+		}
+	}
+
 	@Test
 	public void propertyGenericClassList() throws Exception {
 		IntegerClass genericBean = new IntegerClass();