TypeDescriptor properly narrows ResolvableType for non-typed collection elements

Issue: SPR-14971
(cherry picked from commit aef1460)

Author: jhoeller

spring-core/src/main/java/org/springframework/core/convert/TypeDescriptor.java

@@ -190,7 +190,7 @@ public TypeDescriptor narrow(Object value) {
 			return this;
 		}
 		ResolvableType narrowed = ResolvableType.forType(value.getClass(), getResolvableType());
-		return new TypeDescriptor(narrowed, null, getAnnotations());
+		return new TypeDescriptor(narrowed, value.getClass(), getAnnotations());
 	}
 
 	/**
@@ -439,7 +439,10 @@ private TypeDescriptor narrow(Object value, TypeDescriptor typeDescriptor) {
 		if (typeDescriptor != null) {
 			return typeDescriptor.narrow(value);
 		}
-		return (value != null ? new TypeDescriptor(getResolvableType(), value.getClass(), getAnnotations()) : null);
+		if (value != null) {
+			return narrow(value);
+		}
+		return null;
 	}
 
 	@Override

spring-core/src/test/java/org/springframework/core/convert/converter/DefaultConversionServiceTests.java

@@ -627,13 +627,35 @@ public void convertArrayToArrayAssignable() {
 		assertEquals(3, result[2]);
 	}
 
+	@Test
+	public void convertListOfNonStringifiable() {
+		List<Object> list = Arrays.asList(new TestEntity(1L), new TestEntity(2L));
+		assertTrue(conversionService.canConvert(list.getClass(), String.class));
+		try {
+			conversionService.convert(list, String.class);
+		}
+		catch (ConversionFailedException ex) {
+			assertTrue(ex.getMessage().contains(list.getClass().getName()));
+			assertTrue(ex.getCause() instanceof ConverterNotFoundException);
+			assertTrue(ex.getCause().getMessage().contains(TestEntity.class.getName()));
+		}
+	}
+
+	@Test
+	public void convertListOfStringToString() {
+		List<String> list = Arrays.asList("Foo", "Bar");
+		assertTrue(conversionService.canConvert(list.getClass(), String.class));
+		String result = conversionService.convert(list, String.class);
+		assertEquals("Foo,Bar", result);
+	}
+
 	@Test
 	public void convertListOfListToString() {
 		List<String> list1 = Arrays.asList("Foo", "Bar");
 		List<String> list2 = Arrays.asList("Baz", "Boop");
 		List<List<String>> list = Arrays.asList(list1, list2);
+		assertTrue(conversionService.canConvert(list.getClass(), String.class));
 		String result = conversionService.convert(list, String.class);
-		assertNotNull(result);
 		assertEquals("Foo,Bar,Baz,Boop", result);
 	}