String processing produces code exceeding length limit

[Jackenmen]

Describe the bug

String processing produces code exceeding the length limit when it was already formatted properly before.

To Reproduce

For example:

1. Take this file:

class X:
    async def foo(self):
        msg = ""
        for candidate in CANDIDATES:
            msg += (
                "**{candidate.object_type} {candidate.rev}**"
                " - {candidate.description}\n"
            )

2. Run Black on it with these arguments --experimental-string-processing
3. See the reformatted code that exceeds line length limit:

class X:
    async def foo(self):
        msg = ""
        for candidate in CANDIDATES:
            msg += "**{candidate.object_type} {candidate.rev}** - {candidate.description}\n"

Expected behavior

I expected the code to remain unchanged.

Environment (please complete the following information):

• Version: main (cf75673 at the time of testing)
• OS and Python version: Windows 10/Python 3.8.9

Does this bug also happen on main?

Yes.

Additional context

I was unable to find another issue that would seem to be the same as this though it's possible I missed something. Just to be sure it's not about the same thing #2284 is, I also tried out the changes from #2292 (since they apparently fix some edge cases that weren't fixed by #2286.
Also, the original code actually contained f-string but to rule out that it has anything to do with #2293, I changed it to use regular strings which as you can see from the issue description didn't change the end result.

[Jackenmen]

Probably what's also important here is that Black doesn't try to split this string despite it being too long:

class X:
    async def foo(self):
        msg = ""
        for candidate in CANDIDATES:
            msg += "**{candidate.object_type} {candidate.rev}** - {candidate.description}\n"

After a second look, it appears that usage of += is the key issue here - it works fine when replacing it with =

[JelleZijlstra]

Does #2292 fix this?

[JelleZijlstra]

Never mind, you already discussed it.

[bbugyi200]

This issue should be fixed by the following PR: #2312