Add an option to ignore the mypy version when validating cache metadata (#3641)

Author: gvanrossum

mypy/build.py

@@ -895,7 +895,7 @@ def find_cache_meta(id: str, path: str, manager: BuildManager) -> Optional[Cache
         return None
 
     # Ignore cache if generated by an older mypy version.
-    if (m.version_id != manager.version_id
+    if ((m.version_id != manager.version_id and not manager.options.skip_version_check)
             or m.options is None
             or len(m.dependencies) != len(m.dep_prios)):
         manager.trace('Metadata abandoned for {}: new attributes are missing'.format(id))

mypy/main.py

@@ -289,6 +289,8 @@ def add_invertible_flag(flag: str,
     parser.add_argument('--cache-dir', action='store', metavar='DIR',
                         help="store module cache info in the given folder in incremental mode "
                         "(defaults to '{}')".format(defaults.CACHE_DIR))
+    parser.add_argument('--skip-version-check', action='store_true',
+                        help="allow using cache written by older mypy version")
     add_invertible_flag('--strict-optional', default=False, strict_flag=True,
                         help="enable experimental strict Optional checks")
     parser.add_argument('--strict-optional-whitelist', metavar='GLOB', nargs='*',

mypy/options.py

@@ -113,6 +113,7 @@ def __init__(self) -> None:
         self.cache_dir = defaults.CACHE_DIR
         self.debug_cache = False
         self.quick_and_dirty = False
+        self.skip_version_check = False
 
         # Paths of user plugins
         self.plugins = []  # type: List[str]