Black fails on docstring with linebreak

[DiscoveryFox]

Describe the bug

When you have a docstring and add a \ at the end of the line to get a line break black can't deal with it.

To Reproduce

For example, take this code:

class Database:
    """
    ## Database Base Class
    The custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).
    Provides a lot of functions to interact with the database in a pythonic, easy and
    fast way.
    """
    def __init__(self):
        ...

It works completely fine(im on python version 3.9.7). But when I now add a \ to get a linebreak it stops working

class Database:
    """
    ## Database Base Class
    The custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).\ 
    Provides a lot of functions to interact with the database in a pythonic, easy and
    fast way.
    """
    def __init__(self):
        ...

The resulting error is:

error: cannot format /u/f/fuchsf/Documents/gitlab-audit/tools/database_helper.py: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /var/tmp/blk_jkcx46wd.log

Content of /var/tmp/blk_jkcx46wd.log:

--- src
+++ dst
@@ -2569,11 +2569,11 @@
           value=
             Constant(
               kind=
                 None,  # NoneType
               value=
-                '## Database Base Class\nThe custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).\\\nProvides a lot of functions to interact with the database in a pythonic, easy and\nfast way.',  # str
+                '## Database Base Class\nThe custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).    Provides a lot of functions to interact with the database in a pythonic, easy and\nfast way.',  # str
             )  # /Constant
         )  # /Expr
         FunctionDef(
           args=
             arguments(

Expected behavior

Black usually shouldn't mess with docstrings since they are not part of the source code. And can sometimes be confusing semi-code. It should just work.
Environment

• Black's version: 23.3.0
• OS and Python version: Red Hat Linux with Python 3.9.7 in a Virtualenv

Additional context

Sadly I can't share anymore code since it's from a closed codebase but if there are any more questions I will try to answer as good as I can

[henriholopainen]

Can't repro on current main nor on 23.3.0 running Python 3.9.16.

% black --version                  
black, 23.3.0 (compiled: yes)
Python (CPython) 3.9.16

% black --diff issue_3727.py
--- issue_3727.py	2023-10-29 19:41:26.784549 +0000
+++ issue_3727.py	2023-10-29 19:42:14.183941 +0000
@@ -3,7 +3,8 @@
     ## Database Base Class
     The custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).\
     Provides a lot of functions to interact with the database in a pythonic, easy and
     fast way.
     """
+
     def __init__(self):
         print("foo")
would reformat issue_3727.py

All done! ✨ 🍰 ✨
1 file would be reformatted.

There also seems to be plenty of tests for trailing backslash in docstrings. So maybe this should be closed?

[hauntsaninja]

Interesting, I can still repro on f7cbe4a:

(black-drsy) ~/dev/black main λ python --version         
Python 3.11.5
(black-drsy) ~/dev/black main λ cat test.py
class Database:
    """
    ## Database Base Class
    The custom ORM specificaly created for the [gitlab-audit project](https://gitlab.itwm.fraunhofer.de/it-linux/gitlab-audit).\ 
    Provides a lot of functions to interact with the database in a pythonic, easy and
    fast way.
    """
    def __init__(self):
        ...
(black-drsy) ~/dev/black main λ python -m black test.py --check
error: cannot format test.py: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /var/folders/0y/wt5dx2vj6t16wq97pglrkynm0000gp/T/blk_b16gcy_0.log

Oh no! 💥 💔 💥
1 file would fail to reformat.

[hauntsaninja]

Playground link

[JelleZijlstra]

I can also still reproduce it, both on current main and on 23.3.0 as @henriholopainen tried, and both with and without --preview.

[henriholopainen]

Ok, apparently the issue is a trailing whitespace after the backslash. I can try and take a look at this tomorrow. Thanks for the repro steps all!

[henriholopainen]

While this was a true error, the error was rooted in the trailing whitespace, which probably wasn't intentional. Fix in 4008.