warn_if

[nstarman]

Fixes #761.

I haven't yet cracked why the warnings aren't raised when a function is jitted.

[patrick-kidger]

Let me know if you figure it out!
Other than that -- this has a lot of duplicate code with error_if. Probably this is most easily done with a Boolean flag instead?

[nstarman]

Other than that -- this has a lot of duplicate code with error_if. Probably this is most easily done with a Boolean flag instead?

I was considering that as well, but warn has a number of keyword arguments, like category and stacklevel that aren't shared with raise.
Perhaps the combining could be done at the branched_error_if_impl level, which isn't exposed to the user and thus it's less important if the kwargs are a bit of a mess?

[patrick-kidger]

Yeah, I think that's the right place to do it!

(Although FWIW I think some stuff like stacklevel may be incompatible with JIT? Or maybe you need to just preemptively grab the stack at trace time just in case you want to warn about it later?)