> A new language feature is released, you cannot apply it to old code, since that would make a big PR.
Good. Don't change code for the sake of shiny new things syndrome.
> A better static type checker, that finds some bugs for you, you cannot fix them as your PR would be too big,
Good. Report each bug separately, with a suggested fix, categorised by region of the code. Just because you ran the program, that doesn't mean you understand the code well enough to actually fix stuff: those bugs may be symptomatic of a deeper issue with the module they're part of. The last thing you need is to turn accidentally-correct code into subtly-wrong code.
If you do understand the code well enough, what's the harm in submitting each bugfix as a separate (independent) commit? It makes it easier for the reviewers to go "yup, yup, yup", rather than having to think "does this part affect that part?".
Good. Don't change code for the sake of shiny new things syndrome.
> A better static type checker, that finds some bugs for you, you cannot fix them as your PR would be too big,
Good. Report each bug separately, with a suggested fix, categorised by region of the code. Just because you ran the program, that doesn't mean you understand the code well enough to actually fix stuff: those bugs may be symptomatic of a deeper issue with the module they're part of. The last thing you need is to turn accidentally-correct code into subtly-wrong code.
If you do understand the code well enough, what's the harm in submitting each bugfix as a separate (independent) commit? It makes it easier for the reviewers to go "yup, yup, yup", rather than having to think "does this part affect that part?".