Make String to Boolean parameter conversion more strict

documentation/src/docs/asciidoc/release-notes/release-notes-5.10.0-M1.adoc

@@ -63,6 +63,9 @@ repository on GitHub.
 
 * The `dynamic` parallel execution strategy now allows the thread pool to be saturated by
   default.
+* Implicit type conversion of boolean values like in `@CsvSource` is now stricter, only
+  allowing values `"true"` or `"false"` (case-insensitive), in order to make accidental
+  mistakes apparent, avoiding potential confusion.
 
 ==== New Features and Improvements
 

documentation/src/docs/asciidoc/user-guide/writing-tests.adoc

@@ -1706,7 +1706,7 @@ integral types: `byte`, `short`, `int`, `long`, and their boxed counterparts.
 |===
 | Target Type | Example
 
-| `boolean`/`Boolean`        | `"true"`                                 -> `true`
+| `boolean`/`Boolean`        | `"true"`                                 -> `true` _(only accepts values 'true' or 'false', case-insensitive)_
 | `byte`/`Byte`              | `"15"`, `"0xF"`, or `"017"`              -> `(byte) 15`
 | `char`/`Character`         | `"o"`                                    -> `'o'`
 | `short`/`Short`            | `"15"`, `"0xF"`, or `"017"`              -> `(short) 15`

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/DefaultArgumentConverter.java

@@ -146,7 +146,12 @@ private static class StringToBooleanAndCharPrimitiveConverter implements StringT
 		private static final Map<Class<?>, Function<String, ?>> CONVERTERS;
 		static {
 			Map<Class<?>, Function<String, ?>> converters = new HashMap<>();
-			converters.put(Boolean.class, Boolean::valueOf);
+			converters.put(Boolean.class, source -> {
+				boolean isTrue = "true".equalsIgnoreCase(source);
+				Preconditions.condition(isTrue || "false".equalsIgnoreCase(source),
+					() -> "String must be (ignoring case) 'true' or 'false': " + source);
+				return isTrue;
+			});
 			converters.put(Character.class, source -> {
 				Preconditions.condition(source.length() == 1, () -> "String must have length of 1: " + source);
 				return source.charAt(0);

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

@@ -11,6 +11,7 @@
 package org.junit.jupiter.params.converter;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 import java.io.File;
 import java.lang.Thread.State;
@@ -99,6 +100,21 @@ void convertsStringsToPrimitiveTypes() {
 		assertConverts("42.2_3", double.class, 42.23);
 	}
 
+	@Test
+	void throwsExceptionOnInvalidStringForPrimitiveTypes() {
+		assertThatExceptionOfType(ArgumentConversionException.class) //
+				.isThrownBy(() -> convert("ab", char.class)) //
+				.withMessage("Failed to convert String \"ab\" to type java.lang.Character") //
+				.havingCause() //
+				.withMessage("String must have length of 1: ab");
+
+		assertThatExceptionOfType(ArgumentConversionException.class) //
+				.isThrownBy(() -> convert("tru", boolean.class)) //
+				.withMessage("Failed to convert String \"tru\" to type java.lang.Boolean") //
+				.havingCause() //
+				.withMessage("String must be (ignoring case) 'true' or 'false': tru");
+	}
+
 	/**
 	 * @since 5.4
 	 */
@@ -232,11 +248,14 @@ void convertsStringToUUID() {
 	// -------------------------------------------------------------------------
 
 	private void assertConverts(Object input, Class<?> targetClass, Object expectedOutput) {
-		var result = DefaultArgumentConverter.INSTANCE.convert(input, targetClass);
+		var result = convert(input, targetClass);
 
 		assertThat(result) //
 				.describedAs(input + " --(" + targetClass.getName() + ")--> " + expectedOutput) //
 				.isEqualTo(expectedOutput);
 	}
 
+	private Object convert(Object input, Class<?> targetClass) {
+		return DefaultArgumentConverter.INSTANCE.convert(input, targetClass);
+	}
 }