From e50110353ab81b539aaee686453c18c707b5f045 Mon Sep 17 00:00:00 2001
From: Henri Holopainen <henri.i.holopainen@gmail.com>
Date: Tue, 31 Oct 2023 17:27:11 +0200
Subject: [PATCH] Produce equivalent code for docstrings containing backslash
 followed by whitespace(s) before newline (#4008)

Fixes #3727
---
 CHANGES.md                    |  3 ++-
 src/black/linegen.py          |  3 ++-
 tests/data/cases/docstring.py | 13 +++++++++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index dd5f52cf706..e910fbed162 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -8,7 +8,8 @@
 
 ### Stable style
 
-<!-- Changes that affect Black's stable style -->
+- Fix a crash when whitespace(s) followed a backslash before newline in a docstring
+  (#4008)
 
 ### Preview style
 
diff --git a/src/black/linegen.py b/src/black/linegen.py
index 43bc08efbbd..121c6e314fe 100644
--- a/src/black/linegen.py
+++ b/src/black/linegen.py
@@ -2,6 +2,7 @@
 Generating lines of code.
 """
 
+import re
 import sys
 from dataclasses import replace
 from enum import Enum, auto
@@ -420,7 +421,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
         if Preview.hex_codes_in_unicode_sequences in self.mode:
             normalize_unicode_escape_sequences(leaf)
 
-        if is_docstring(leaf) and "\\\n" not in leaf.value:
+        if is_docstring(leaf) and not re.search(r"\\\s*\n", leaf.value):
             # We're ignoring docstrings with backslash newline escapes because changing
             # indentation of those changes the AST representation of the code.
             if self.mode.string_normalization:
diff --git a/tests/data/cases/docstring.py b/tests/data/cases/docstring.py
index c31d6a68783..e983c5bd438 100644
--- a/tests/data/cases/docstring.py
+++ b/tests/data/cases/docstring.py
@@ -221,6 +221,12 @@ def stable_quote_normalization_with_immediate_inner_single_quote(self):
     '''
 
 
+def foo():
+    """
+    Docstring with a backslash followed by a space\ 
+    and then another line
+    """
+
 # output
 
 class MyClass:
@@ -442,3 +448,10 @@ def stable_quote_normalization_with_immediate_inner_single_quote(self):
 
     <text here, since without another non-empty line black is stable>
     """
+
+
+def foo():
+    """
+    Docstring with a backslash followed by a space\ 
+    and then another line
+    """