Enhance canConvert with inner converters evaluation

Author: scordio

junit-platform-commons/src/main/java/org/junit/platform/commons/support/conversion/DefaultConversionService.java

@@ -78,7 +78,20 @@ private DefaultConversionService() {
 	 */
 	@Override
 	public boolean canConvert(Object source, Class<?> targetType, ClassLoader classLoader) {
-		return source instanceof String;
+		if (source == null && targetType.isPrimitive()) {
+			return false;
+		}
+
+		if (!(source instanceof String)) {
+			return false;
+		}
+
+		if (String.class.equals(targetType)) {
+			return true;
+		}
+
+		return stringToObjectConverters.stream().anyMatch(
+			candidate -> candidate.canConvertTo(toWrapperType(targetType)));
 	}
 
 	/**
@@ -145,7 +158,6 @@ public Object convert(Object source, Class<?> targetType, ClassLoader classLoade
 			return source;
 		}
 
-		// FIXME move/copy next three lines to canConvert?
 		Class<?> targetTypeToUse = toWrapperType(targetType);
 		Optional<StringToObjectConverter> converter = stringToObjectConverters.stream().filter(
 			candidate -> candidate.canConvertTo(targetTypeToUse)).findFirst();

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

@@ -186,7 +186,7 @@ void throwsExceptionOnInvalidStringForPrimitiveTypes() {
 	void throwsExceptionWhenImplicitConverstionIsUnsupported() {
 		assertThatExceptionOfType(ArgumentConversionException.class) //
 				.isThrownBy(() -> convert("foo", Enigma.class)) //
-				.withMessage("No built-in converter for source type java.lang.String and target type %s", // FIXME enhance DefaultConversionService::canConvert
+				.withMessage("No registered or built-in converter for source type java.lang.String and target type %s",
 					Enigma.class.getName());
 
 		assertThatExceptionOfType(ArgumentConversionException.class) //