Fix #34760: ignore Kotlin DefaultConstructorMarker in BeanUtils#getParameterNames

[Emil-Stampfly-He]

Issue#34760

Description

When a Kotlin value class is used as a constructor parameter, the compiler generates an extra synthetic parameter of type kotlin.jvm.internal.DefaultConstructorMarker.

When this step is executed:

@Nullable String[] paramNames = (cp != null ? cp.value() : parameterNameDiscoverer.getParameterNames(ctor));

It will call KotlinReflectionParameterNameDiscoverer#getParameterNames(List<KParameter>), where ReflectJvmMapping#getKotlinFunction is called to convert a constructor into a function. DefaultConstructorMarker will be dropped after the conversion. This then causes a mismatch between the number of parameter names discovered and the actual constructor parameter count in BeanUtils#getParameterNames:

Assert.state(paramNames.length == ctor.getParametersCount(),
            () -> "Invalid number of parameter names: " + paramNames.length + " for constructor " + ctor);

Since DefaultConstructorMarker exists solely to prevent signature clashes, it is safe to exclude it from the parameter count. Similar thought as in: Ignore binding Kotlin synthetic default constructors

Proposed Change

Replace the existing assertion with:

long realParamsCount = Arrays.stream(ctor.getParameters())
        .filter(p -> !DefaultConstructorMarker.class.equals(p.getType()))
        .count();
Assert.state(paramNames.length == realParamsCount,
        () -> "Invalid number of parameter names: " + paramNames.length + " for constructor " + ctor);

Tests

Added new cases to BeanUtilsKotlinTests to test BeanUtils#getParameterNames.