Fix "attribute 'arguments' of 'FuncDef' undefined" incremental crash (p…

…ython#12324)

When deserializing from cache, FuncDef.arguments is not set, so
check before use.

mypy/messages.py

@@ -1978,7 +1978,9 @@ def [T <: int] f(self, x: int, y: T) -> None
             s += ' = ...'
 
     # If we got a "special arg" (i.e: self, cls, etc...), prepend it to the arg list
-    if isinstance(tp.definition, FuncDef) and tp.definition.name is not None:
+    if (isinstance(tp.definition, FuncDef) and
+            tp.definition.name is not None and
+            hasattr(tp.definition, 'arguments')):
         definition_args = [arg.variable.name for arg in tp.definition.arguments]
         if definition_args and tp.arg_names != definition_args \
                 and len(definition_args) > 0 and definition_args[0]:

mypy/nodes.py

@@ -658,7 +658,7 @@ def set_line(self,
 class FuncItem(FuncBase):
     """Base class for nodes usable as overloaded function items."""
 
-    __slots__ = ('arguments',  # Note that can be None if deserialized (type is a lie!)
+    __slots__ = ('arguments',  # Note that can be unset if deserialized (type is a lie!)
                  'arg_names',  # Names of arguments
                  'arg_kinds',  # Kinds of arguments
                  'min_args',  # Minimum number of arguments

mypy/types.py

@@ -1524,16 +1524,15 @@ def __init__(self,
             # after serialization, but it is useful in error messages.
             # TODO: decide how to add more info here (file, line, column)
             # without changing interface hash.
-            self.def_extras = {
-                'first_arg': (
-                    definition.arguments[0].variable.name
-                    if (getattr(definition, 'arguments', None)
-                        and definition.arg_names
-                        and definition.info
-                        and not definition.is_static)
-                    else None
-                ),
-            }
+            first_arg: Optional[str] = None
+            if (definition.arg_names and
+                    definition.info and
+                    not definition.is_static):
+                if getattr(definition, 'arguments', None):
+                    first_arg = definition.arguments[0].variable.name
+                else:
+                    first_arg = definition.arg_names[0]
+            self.def_extras = {'first_arg': first_arg}
         else:
             self.def_extras = {}
         self.type_guard = type_guard

test-data/unit/check-modules.test

@@ -3212,3 +3212,35 @@ from dir1 import *
 from .test2 import *
 [file dir1/test2.py]
 from test1 import aaaa  # E: Module "test1" has no attribute "aaaa"
+
+[case testIncompatibleOverrideFromCachedModuleIncremental]
+import b
+[file a.py]
+class Foo:
+    def frobnicate(self, *args, **kwargs): pass
+[file b.py]
+from a import Foo
+class Bar(Foo):
+    def frobnicate(self) -> None: pass
+[file b.py.2]
+from a import Foo
+class Bar(Foo):
+    def frobnicate(self, *args) -> None: pass
+[file b.py.3]
+from a import Foo
+class Bar(Foo):
+    def frobnicate(self, *args) -> None: pass # type: ignore[override] # I know
+[builtins fixtures/tuple.pyi]
+[builtins fixtures/dict.pyi]
+[out1]
+tmp/b.py:3: error: Signature of "frobnicate" incompatible with supertype "Foo"
+tmp/b.py:3: note:      Superclass:
+tmp/b.py:3: note:          def frobnicate(self, *args: Any, **kwargs: Any) -> Any
+tmp/b.py:3: note:      Subclass:
+tmp/b.py:3: note:          def frobnicate(self) -> None
+[out2]
+tmp/b.py:3: error: Signature of "frobnicate" incompatible with supertype "Foo"
+tmp/b.py:3: note:      Superclass:
+tmp/b.py:3: note:          def frobnicate(self, *args: Any, **kwargs: Any) -> Any
+tmp/b.py:3: note:      Subclass:
+tmp/b.py:3: note:          def frobnicate(self, *args: Any) -> None