static Class<?> kFunctionReturnType(Method method) {
        assert K_FUNCTION_RETURN_TYPE != null;
        try {
            return (Class<?>) K_FUNCTION_RETURN_TYPE.invoke(null, method);
        } catch (Exception e) {
            return Exceptions.throwUnsafely(e);
        }
    }
{@link Method#getReturnType} equivalent for Kotlin functions.
core/src/main/java/com/linecorp/armeria/internal/server/annotation/KotlinUtil.java