Calculate implicit metaclasses (#2819)

Fix an issue in #2475 - metaclasses are not calculated for subclasses (this 
is needed for Enum).

Author: elazarg

mypy/checkmember.py

@@ -134,7 +134,7 @@ def analyze_member_access(name: str,
             if not is_operator:
                 # When Python sees an operator (eg `3 == 4`), it automatically translates that
                 # into something like `int.__eq__(3, 4)` instead of `(3).__eq__(4)` as an
-                # optimation.
+                # optimization.
                 #
                 # While it normally it doesn't matter which of the two versions are used, it
                 # does cause inconsistencies when working with classes. For example, translating

mypy/nodes.py

@@ -2017,6 +2017,8 @@ def calculate_metaclass_type(self) -> 'Optional[mypy.types.Instance]':
             return mypy.types.Instance(self, [])
         candidates = [s.declared_metaclass for s in self.mro if s.declared_metaclass is not None]
         for c in candidates:
+            if c.type.mro is None:
+                continue
             if all(other.type in c.type.mro for other in candidates):
                 return c
         return None

mypy/semanal.py

@@ -969,10 +969,11 @@ def analyze_metaclass(self, defn: ClassDef) -> None:
             inst = fill_typevars(sym.node)
             assert isinstance(inst, Instance)
             defn.info.declared_metaclass = inst
-            defn.info.metaclass_type = defn.info.calculate_metaclass_type()
-            if defn.info.metaclass_type is None:
-                # Inconsistency may happen due to multiple baseclasses even in classes that
-                # do not declare explicit metaclass, but it's harder to catch at this stage
+        defn.info.metaclass_type = defn.info.calculate_metaclass_type()
+        if defn.info.metaclass_type is None:
+            # Inconsistency may happen due to multiple baseclasses even in classes that
+            # do not declare explicit metaclass, but it's harder to catch at this stage
+            if defn.metaclass:
                 self.fail("Inconsistent metaclass structure for '%s'" % defn.name, defn)
 
     def object_type(self) -> Instance:

test-data/unit/check-classes.test

@@ -2785,6 +2785,18 @@ def f(TA: Type[A]):
     reveal_type(TA)  # E: Revealed type is 'Type[__main__.A]'
     reveal_type(TA.x)  # E: Revealed type is 'builtins.int'
 
+[case testMetaclassSubclass]
+from typing import Type
+class M(type):
+    x = 0  # type: int
+
+class A(metaclass=M): pass
+class B(A): pass
+
+def f(TB: Type[B]):
+    reveal_type(TB)  # E: Revealed type is 'Type[__main__.B]'
+    reveal_type(TB.x)  # E: Revealed type is 'builtins.int'
+
 [case testMetaclassIterable]
 from typing import Iterable, Iterator