Is it bad idea to add new functions into an existing 3rd party library's namespace?

My code is using a library called "tf2" and it has a namespace of tf2 and overloaded functions like tf2:fromMsg().

Then in my code, I would like to add one more overloaded function tf2:fromMsg() but with difference parameters.

So I write code like this: (in my own code)

namespace tf2
{
void fromMsg(const geometry_msgs::msg::PoseStamped & msg, tf2::Stamped<tf2::Transform> & out)
{
  ...
}
}  // namespace tf2

I see some benefits, so that the user of my code use familiar tf:frmMsg() like other code which already uses tf:frmMsg().

But I also see some concerns: the code is actually hacking into the existing namespace and pretending it's a whole part but actually it's not.

So, is it good or bad?

Bad idea.

At some point in the future, it may violate the "One Definition Rule" and lead to undefined behavior.

You can use the using clause to help simplify things:

namespace tf2Extension
{
void fromMsg(const geometry_msgs::msg::PoseStamped & msg, tf2::Stamped<tf2::Transform> & out)
{
  ...
}
}

.....

void myFunction()
{
    using  tf2Extension::fromMsg;
    using  tf2::fromMsg;

    fromMsg(<Parameters That distinguish usage>);
}

Now the compiler will tell you when the tf2 library is extended and clashes with your function.