Test 5.9.x behavior for null/"null" conversion support in parameterized tests

2 tests in this commit currently fail due to changes in 5.10 with the
failure message:

Failed to convert String "null" to type java.lang.Boolean

This will be addressed in a subsequent commit to align with the
expected behavior in 5.10.x.

See #3472

Author: sbrannen

junit-jupiter-params/src/test/java/org/junit/jupiter/params/ParameterizedTestIntegrationTests.java

@@ -111,6 +111,40 @@
  */
 class ParameterizedTestIntegrationTests {
 
+	@ParameterizedTest
+	@CsvSource(textBlock = """
+			apple,   True
+			banana,  true
+			lemon,   false
+			kumquat, null
+			""")
+	void sweetFruit(String fruit, Boolean sweet) {
+		switch (fruit) {
+			case "apple" -> assertThat(sweet).isTrue();
+			case "banana" -> assertThat(sweet).isTrue();
+			case "lemon" -> assertThat(sweet).isFalse();
+			case "kumquat" -> assertThat(sweet).isFalse(); // "null" --> false
+			default -> fail("Unexpected fruit : " + fruit);
+		}
+	}
+
+	@ParameterizedTest
+	@CsvSource(nullValues = "null", textBlock = """
+			apple,   True
+			banana,  true
+			lemon,   false
+			kumquat, null
+			""")
+	void sweetFruitWithNullableBoolean(String fruit, Boolean sweet) {
+		switch (fruit) {
+			case "apple" -> assertThat(sweet).isTrue();
+			case "banana" -> assertThat(sweet).isTrue();
+			case "lemon" -> assertThat(sweet).isFalse();
+			case "kumquat" -> assertThat(sweet).isNull(); // null --> null
+			default -> fail("Unexpected fruit : " + fruit);
+		}
+	}
+
 	@ParameterizedTest
 	@CsvSource(quoteCharacter = '"', textBlock = """
 

junit-jupiter-params/src/test/java/org/junit/jupiter/params/converter/DefaultArgumentConverterTests.java

@@ -47,6 +47,8 @@
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.junit.platform.commons.test.TestClassLoader;
 import org.junit.platform.commons.util.ReflectionUtils;
 
@@ -61,11 +63,13 @@ class DefaultArgumentConverterTests {
 	void isAwareOfNull() {
 		assertConverts(null, Object.class, null);
 		assertConverts(null, String.class, null);
+		assertConverts(null, Boolean.class, null);
 	}
 
 	@Test
 	void isAwareOfWrapperTypesForPrimitiveTypes() {
 		assertConverts(true, boolean.class, true);
+		assertConverts(false, boolean.class, false);
 		assertConverts((byte) 1, byte.class, (byte) 1);
 		assertConverts('o', char.class, 'o');
 		assertConverts((short) 1, short.class, (short) 1);
@@ -91,6 +95,7 @@ void isAwareOfWideningConversions() {
 	@Test
 	void convertsStringsToPrimitiveTypes() {
 		assertConverts("true", boolean.class, true);
+		assertConverts("false", boolean.class, false);
 		assertConverts("o", char.class, 'o');
 		assertConverts("1", byte.class, (byte) 1);
 		assertConverts("1_0", byte.class, (byte) 10);
@@ -106,6 +111,54 @@ void convertsStringsToPrimitiveTypes() {
 		assertConverts("42.2_3", double.class, 42.23);
 	}
 
+	@Test
+	void convertsStringsToPrimitiveWrapperTypes() {
+		assertConverts("true", Boolean.class, true);
+		assertConverts("false", Boolean.class, false);
+		assertConverts("o", Character.class, 'o');
+		assertConverts("1", Byte.class, (byte) 1);
+		assertConverts("1_0", Byte.class, (byte) 10);
+		assertConverts("1", Short.class, (short) 1);
+		assertConverts("1_2", Short.class, (short) 12);
+		assertConverts("42", Integer.class, 42);
+		assertConverts("700_050_000", Integer.class, 700_050_000);
+		assertConverts("42", Long.class, 42L);
+		assertConverts("4_2", Long.class, 42L);
+		assertConverts("42.23", Float.class, 42.23f);
+		assertConverts("42.2_3", Float.class, 42.23f);
+		assertConverts("42.23", Double.class, 42.23);
+		assertConverts("42.2_3", Double.class, 42.23);
+	}
+
+	@Test
+	void convertsTheWordNullToBooleanFalse() {
+		assertConverts("null", boolean.class, false);
+		assertConverts("NULL", boolean.class, false);
+		assertConverts("null", Boolean.class, false);
+		assertConverts("NULL", Boolean.class, false);
+	}
+
+	@ParameterizedTest(name = "[{index}] {0}")
+	@ValueSource(classes = { char.class, boolean.class, short.class, byte.class, int.class, long.class, float.class,
+			double.class })
+	void throwsExceptionForNullToPrimitiveTypeConversion(Class<?> type) {
+		assertThatExceptionOfType(ArgumentConversionException.class) //
+				.isThrownBy(() -> convert(null, type)) //
+				.withMessage("Cannot convert null to primitive value of type " + type.getCanonicalName());
+	}
+
+	@ParameterizedTest(name = "[{index}] {0}")
+	// NOTE: everything except Boolean.class and Character.class.
+	@ValueSource(classes = { Short.class, Byte.class, Integer.class, Long.class, Float.class, Double.class })
+	void throwsExceptionWhenConvertingTheWordNullToPrimitiveWrapperType(Class<?> type) {
+		assertThatExceptionOfType(ArgumentConversionException.class) //
+				.isThrownBy(() -> convert("null", type)) //
+				.withMessage("Failed to convert String \"null\" to type " + type.getCanonicalName());
+		assertThatExceptionOfType(ArgumentConversionException.class) //
+				.isThrownBy(() -> convert("NULL", type)) //
+				.withMessage("Failed to convert String \"NULL\" to type " + type.getCanonicalName());
+	}
+
 	@Test
 	void throwsExceptionOnInvalidStringForPrimitiveTypes() {
 		assertThatExceptionOfType(ArgumentConversionException.class) //

junit-jupiter-params/src/test/java/org/junit/jupiter/params/provider/CsvArgumentsProviderTests.java

@@ -248,11 +248,12 @@ void customEmptyValueAndDefaultNullValue() {
 
 	@Test
 	void customNullValues() {
-		var annotation = csvSource().nullValues("N/A", "NIL").lines("apple, , NIL, '', N/A, banana").build();
+		var annotation = csvSource().nullValues("N/A", "NIL", "null")//
+				.lines("apple, , NIL, '', N/A, banana, null").build();
 
 		var arguments = provideArguments(annotation);
 
-		assertThat(arguments).containsExactly(array("apple", null, null, "", null, "banana"));
+		assertThat(arguments).containsExactly(array("apple", null, null, "", null, "banana", null));
 	}
 
 	@Test