Fix is_overlapping_types for Any (#2973)

`Any` is now properly special-cased to potentially overlap with all other types.  Fixes #2970.

Author: benkuhn

mypy/meet.py

@@ -69,6 +69,10 @@ class C(A, B): ...
     TODO: Don't consider callables always overlapping.
     TODO: Don't consider type variables with values always overlapping.
     """
+    # Any overlaps with everything
+    if isinstance(t, AnyType) or isinstance(s, AnyType):
+        return True
+
     # Since we are effectively working with the erased types, we only
     # need to handle occurrences of TypeVarType at the top level.
     if isinstance(t, TypeVarType):

test-data/unit/check-optional.test

@@ -77,6 +77,15 @@ else:
   reveal_type(x)  # E: Revealed type is 'builtins.int'
 [builtins fixtures/bool.pyi]
 
+[case testAnyCanBeNone]
+from typing import Optional, Any
+x = None  # type:  Any
+if x is None:
+  reveal_type(x)  # E: Revealed type is 'builtins.None'
+else:
+  reveal_type(x)  # E: Revealed type is 'Any'
+[builtins fixtures/bool.pyi]
+
 [case testOrCases]
 from typing import Optional
 x = None  # type: Optional[str]