Learn that loop condition is False on exit from while loop

mypy/checker.py

@@ -226,8 +226,10 @@ def accept(self, node: Node, type_context: Type = None) -> Type:
         else:
             return typ
 
-    def accept_loop(self, body: Node, else_body: Node = None) -> Type:
+    def accept_loop(self, body: Node, else_body: Node = None, *,
+                    exit_condition: Node = None) -> Type:
         """Repeatedly type check a loop body until the frame doesn't change.
+        If exit_condition is set, assume it must be False on exit from the loop.
 
         Then check the else_body.
         """
@@ -240,6 +242,13 @@ def accept_loop(self, body: Node, else_body: Node = None) -> Type:
                 if not self.binder.last_pop_changed:
                     break
             self.binder.pop_loop_frame()
+            if exit_condition:
+                _, else_map = find_isinstance_check(
+                    exit_condition, self.type_map, self.typing_mode_weak()
+                )
+                if else_map:
+                    for var, type in else_map.items():
+                        self.binder.push(var, type)
             if else_body:
                 self.accept(else_body)
 
@@ -1465,7 +1474,8 @@ def visit_if_stmt(self, s: IfStmt) -> Type:
 
     def visit_while_stmt(self, s: WhileStmt) -> Type:
         """Type check a while statement."""
-        self.accept_loop(IfStmt([s.expr], [s.body], None), s.else_body)
+        self.accept_loop(IfStmt([s.expr], [s.body], None), s.else_body,
+                         exit_condition=s.expr)
 
     def visit_operator_assignment_stmt(self,
                                        s: OperatorAssignmentStmt) -> Type:

test-data/unit/check-isinstance.test

@@ -908,6 +908,42 @@ def bar() -> None:
 [out]
 main: note: In function "bar":
 
+[case testWhileExitCondition1]
+from typing import Union
+x = 1  # type: Union[int, str]
+while isinstance(x, int):
+    if bool():
+        continue
+    x = 'a'
+else:
+    reveal_type(x)  # E: Revealed type is 'builtins.str'
+reveal_type(x)  # E: Revealed type is 'builtins.str'
+[builtins fixtures/isinstance.py]
+
+[case testWhileExitCondition2]
+from typing import Union
+x = 1  # type: Union[int, str]
+while isinstance(x, int):
+    if bool():
+        break
+    x = 'a'
+else:
+    reveal_type(x)  # E: Revealed type is 'builtins.str'
+reveal_type(x)  # E: Revealed type is 'Union[builtins.int, builtins.str]'
+[builtins fixtures/isinstance.py]
+
+[case testWhileLinkedList]
+from typing import Union
+LinkedList = Union['Cons', 'Nil']
+class Nil: pass
+class Cons:
+    tail = None  # type: LinkedList
+def last(x: LinkedList) -> Nil:
+    while isinstance(x, Cons):
+        x = x.tail
+    return x
+[builtins fixtures/isinstance.py]
+
 [case testReturnAndFlow]
 def foo() -> int:
     return 1 and 2