Which platform define did you use?

Some arduino SDKs do define a new operator and others don't. This is currently handled by the feature-define USTD_FEATURE_SUPPORTS_NEW_OPERATOR, so that platforms that have a new operator implementation simply define this.
Making this inline effectively causes two different new implementations without the compiler noticing, so that could be problematic.

Hi, I'm using platformio + Arduino nano, which does not have that operator defined.

The issue is that if you have two cpp files (a.cpp and b.cpp) that both include functional.h you'll end up with two instances of that operator implementation causing a linker error.

Defining that function inline prevents that. An alternative would be to just have the prototype in the header file and move the implementation to a c file.

Hmm. That's a valid point.
A hacky solution would be to #define USTD_FEATURE_SUPPORTS_NEW_OPERATOR in one of the cpp files before including ustd_functional.h.
Need to think about this more.

Unfortunately that hack does not work, as there is no definition of that function in Arduino.h or something. So then that implementation would exist but there is no header definition for that :/

I guess you're right, it's going to be inline.
This problem also affects other global functions e.g. in ustd_platform.h.
I've to investigate the extend a bit more...

Release 0.6.2 is published in platformio, thank you!

Thank you very much, greatly appreciated!