Revisit the body of a loop if the number of partial types has changed.

mypy/checker.py

@@ -607,11 +607,14 @@ def accept_loop(
         """
         # The outer frame accumulates the results of all iterations
         with self.binder.frame_context(can_skip=False, conditional_frame=True):
+            partials_old = sum(len(pts.map) for pts in self.partial_types)
             while True:
                 with self.binder.frame_context(can_skip=True, break_frame=2, continue_frame=1):
                     self.accept(body)
-                if not self.binder.last_pop_changed:
+                partials_new = sum(len(pts.map) for pts in self.partial_types)
+                if (partials_new == partials_old) and not self.binder.last_pop_changed:
                     break
+                partials_old = partials_new
             if exit_condition:
                 _, else_map = self.find_isinstance_check(exit_condition)
                 self.push_type_map(else_map)

mypyc/test-data/commandline.test

@@ -200,9 +200,9 @@ wtvr = next(i for i in range(10) if i == 5)
 
 d1 = {1: 2}
 
-# Make sure we can produce an error when we hit the awful None case
+# Since PR 18180, the following pattern should pose no problems anymore:
 def f(l: List[object]) -> None:
-    x = None  # E: Local variable "x" has inferred type None; add an annotation
+    x = None
     for i in l:
         if x is None:
             x = i

test-data/unit/check-narrowing.test

@@ -2352,3 +2352,40 @@ def fn_while(arg: T) -> None:
         return None
     return None
 [builtins fixtures/primitives.pyi]
+
+[case testRefinePartialTypeWithinLoop]
+
+x = None
+for _ in range(2):
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    x = 1
+reveal_type(x)  # N: Revealed type is "Union[builtins.int, None]"
+
+def f() -> bool: ...
+
+y = None
+while f():
+    reveal_type(y)  # N: Revealed type is "None" \
+                    # N: Revealed type is "Union[builtins.int, None]"
+    y = 1
+reveal_type(y)  # N: Revealed type is "Union[builtins.int, None]"
+
+z = []  # E: Need type annotation for "z" (hint: "z: List[<type>] = ...")
+def g() -> None:
+    for i in range(2):
+        while f():
+            if z:
+                z[0] + "v"  # E: Unsupported operand types for + ("int" and "str")
+            z.append(1)
+
+class A:
+    def g(self) -> None:
+        z = []  # E: Need type annotation for "z" (hint: "z: List[<type>] = ...")
+        for i in range(2):
+            while f():
+                if z:
+                    z[0] + "v"  # E: Unsupported operand types for + ("int" and "str")
+                z.append(1)
+
+[builtins fixtures/primitives.pyi]

test-data/unit/fixtures/primitives.pyi

@@ -48,6 +48,7 @@ class memoryview(Sequence[int]):
 class tuple(Generic[T]):
     def __contains__(self, other: object) -> bool: pass
 class list(Sequence[T]):
+    def append(self, v: T) -> None: pass
     def __iter__(self) -> Iterator[T]: pass
     def __contains__(self, other: object) -> bool: pass
     def __getitem__(self, item: int) -> T: pass