Python and Javascript (in the browser) due to their single threaded nature. C++ too as long as you have a std::atomic on the left hand side (since they overload the operator).

it has been in Python due to the GIL.

It's not atomic even with the GIL, though: another thread can run in between the bytecode's load and increment, right?

The GIL's guarantees didn't extend to this.

There is NB_INPLACE_ADD... but I'm struggling to find enough details to be truly confident :\ possibly its existence is misleading other people (thus me) to think += is a single operation in bytecode.

Or, on further reading, maybe it applies to anything that implements `_iadd_` in C. Which does not appear to include native longs: https://github.com/python/cpython/blob/main/Objects/longobje...

no... but some languages may disallow simultaneously holding a reference in different execution threads