Walrus operators unrecognised in dictionary initialisation of a class attribute

It seems Walrus operators (in Python 3.8+) are not recognised by mypy when used in initialising a dictionary of a class attribute. Here is a test code.

class TryWalrus:
    CKEY = { 
        'KY': (p:='a_'),
         'ANOTHER': p+'content',
        }

print(TryWalrus.CKEY['ANOTHER']) # => a_content

This outputs the following:

$ mypy try_walrus.py
try_walrus.py:4: error: Name 'p' is not defined
Found 1 error in 1 file (checked 1 source file)

If the variable CKEY is out of the class definition, i.e. if directly in Main, it does not raise an error.

I tested it with Python 3.8.1 and mypy version 0.770+dev.8888b1aedf6d4ba543ae12f538f190ce5e65c20d (yesterday's head).

Start with the reproducer in try_walrus.py and run mypy against it using the class-level dictionary shown in the report. Trace how walrus assignments are processed inside class attribute initialisation, then confirm that the valid Python 3.8 example no longer reports Name 'p' as undefined.