Re-broaden type to Any if it started as Any but was narrowed in an enclosing frame (#3361)

Fixes #3338.

Author: lincolnq

mypy/binder.py

@@ -233,12 +233,16 @@ def assign_type(self, expr: Expression,
             # times?
             return
 
-        # If x is Any and y is int, after x = y we do not infer that x is int.
-        # This could be changed.
-
-        if (isinstance(self.most_recent_enclosing_type(expr, type), AnyType)
+        enclosing_type = self.most_recent_enclosing_type(expr, type)
+        if (isinstance(enclosing_type, AnyType)
                 and not restrict_any):
-            pass
+            # If x is Any and y is int, after x = y we do not infer that x is int.
+            # This could be changed.
+            if not isinstance(type, AnyType):
+                # We narrowed type from Any in a recent frame (probably an
+                # isinstance check), but now it is reassigned, so broaden back
+                # to Any (which is the most recent enclosing type)
+                self.put(expr, enclosing_type)
         elif (isinstance(type, AnyType)
               and not (isinstance(declared_type, UnionType)
                        and any(isinstance(item, AnyType) for item in declared_type.items))):

test-data/unit/check-isinstance.test

@@ -1754,3 +1754,17 @@ if isinstance(x, str, 1): # E: Too many arguments for "isinstance"
     x = 1
     reveal_type(x) # E: Revealed type is 'builtins.int'
 [builtins fixtures/isinstancelist.pyi]
+
+
+[case testIsinstanceNarrowAny]
+from typing import Any
+
+def narrow_any_to_str_then_reassign_to_int() -> None:
+    v = 1 # type: Any
+
+    if isinstance(v, str):
+        reveal_type(v)  # E: Revealed type is 'builtins.str'
+        v = 2
+        reveal_type(v)  # E: Revealed type is 'Any'
+
+[builtins fixtures/isinstance.pyi]