Account for += assignment when deciding whether to split string

Fixes psf#2294

src/black/trans.py

@@ -867,7 +867,7 @@ def _get_max_string_length(self, line: Line, string_idx: int) -> int:
                 # WMA4 a space, a comma, and a closing bracket [e.g. `), STRING`].
                 offset += 3
 
-            if P.type in [token.COLON, token.EQUAL, token.NAME]:
+            if P.type in [token.COLON, token.EQUAL, token.PLUSEQUAL, token.NAME]:
                 # This conditional branch is meant to handle dictionary keys,
                 # variable assignments, 'return STRING' statement lines, and
                 # 'else STRING' ternary expression lines.

tests/data/long_strings__edge_case.py

@@ -32,6 +32,9 @@
 assert str(result) == "This long string should be split at some point right close to or around hereeeeeee"
 assert str(result) < "This long string should be split at some point right close to or around hereeeeee"
 assert "A format string: %s" % "This long string should be split at some point right close to or around hereeeeeee" != result
+msg += "This long string should be split at some point right close to or around hereeeee"
+msg += "This long string should be split at some point right close to or around hereeeeeeee"
+msg += "This long string should not be split at any point ever since it is just righttt"
 
 
 # output
@@ -127,3 +130,11 @@
     " hereeeeeee"
     != result
 )
+msg += (
+    "This long string should be split at some point right close to or around hereeeee"
+)
+msg += (
+    "This long string should be split at some point right close to or around"
+    " hereeeeeeee"
+)
+msg += "This long string should not be split at any point ever since it is just righttt"

tests/data/long_strings__regression.py

@@ -407,6 +407,16 @@ def _legacy_listen_examples():
     )
 
 
+class X:
+    async def foo(self):
+        msg = ""
+        for candidate in CANDIDATES:
+            msg += (
+                "**{candidate.object_type} {candidate.rev}**"
+                " - {candidate.description}\n"
+            )
+
+
 temp_msg = (
     f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
     f"{balance: <{bal_len + 5}} "
@@ -948,6 +958,16 @@ def _legacy_listen_examples():
     )
 
 
+class X:
+    async def foo(self):
+        msg = ""
+        for candidate in CANDIDATES:
+            msg += (
+                "**{candidate.object_type} {candidate.rev}**"
+                " - {candidate.description}\n"
+            )
+
+
 temp_msg = (
     f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
     f"{balance: <{bal_len + 5}} "