Black removes indentation in multi-line dict with multi-line value

[gsamaras]

Describe the bug

A code snippet is worth a thousand words.

To Reproduce Steps to reproduce the behavior:

It removes the indentation from the second line of a two line long value string:

mydict = {
    'key0': 'value0',
    'key1': 'long-two-lines-string-value1-does-not-fit-in-one-line-has-to-continue'
            'value1'
}

to:

mydict = {
    'key0': 'value0',
    'key1': 'long-two-lines-string-value1-does-not-fit-in-one-line-has-to-continue'
    'value1',
}

Demo

Expected behavior
Enforce/keep the 2nd line of value to start its indentation from same column as 1st line of value.

The wrapped element, which should follow the guidance in PEP8 -- Style Guide for Python Code#indentation, is the value.

Environment (please complete the following information):

• Version: master
• OS and Python version: macOS, Python 3.6.x

Additional context

Stack Overflow: PEP8 multi-line dict with multi-line value.

[textbook]

Related: #620

[JelleZijlstra]

The --experimental-string-processing flag fixes this example:

 % black --experimental-string-processing --diff issue1758.py
--- issue1758.py	2021-05-29 02:55:21.422319 +0000
+++ issue1758.py	2021-05-29 02:55:32.992736 +0000
@@ -1,5 +1,6 @@
 mydict = {
-    'key0': 'value0',
-    'key1': 'long-two-lines-string-value1-does-not-fit-in-one-line-has-to-continue'
-            'value1'
+    "key0": "value0",
+    "key1": (
+        "long-two-lines-string-value1-does-not-fit-in-one-line-has-to-continuevalue1"
+    ),
 }
would reformat issue1758.py
All done! ✨ 🍰 ✨
1 file would be reformatted.

[JelleZijlstra]

Duplicate of #620