Fix bug in multiassign from union

Author: ilevkivskyi

mypy/checker.py

@@ -4001,7 +4001,7 @@ def check_multi_assignment_from_union(
                 for t, lv in zip(transposed, self.flatten_lvalues(lvalues)):
                     # We can access _type_maps directly since temporary type maps are
                     # only created within expressions.
-                    t.append(self._type_maps[0].pop(lv, AnyType(TypeOfAny.special_form)))
+                    t.append(self._type_maps[-1].pop(lv, AnyType(TypeOfAny.special_form)))
         union_types = tuple(make_simplified_union(col) for col in transposed)
         for expr, items in assignments.items():
             # Bind a union of types collected in 'assignments' to every expression.

mypy/checkexpr.py

@@ -5993,7 +5993,9 @@ def accept(
             # Deeply nested generic calls can deteriorate performance dramatically.
             # Although in most cases caching makes little difference, in worst case
             # it avoids exponential complexity.
-            # TODO: figure out why caching within lambdas is fragile.
+            # We cannot use cache inside lambdas, because they skip immediate type
+            # context, and use enclosing one, see infer_lambda_type_using_context().
+            # TODO: consider using cache for more expression kinds.
             elif isinstance(node, (CallExpr, ListExpr, TupleExpr)) and not (
                 self.in_lambda_expr or self.chk.current_node_deferred
             ):

test-data/unit/check-overloading.test

@@ -6801,3 +6801,26 @@ class D(Generic[T]):
 a: D[str]  # E: Type argument "str" of "D" must be a subtype of "C"
 reveal_type(a.f(1))  # N: Revealed type is "builtins.int"
 reveal_type(a.f("x"))  # N: Revealed type is "builtins.str"
+
+[case testMultiAssignFromUnionInOverloadCached]
+from typing import Iterable, overload, Union, Optional
+
+@overload
+def always_bytes(str_or_bytes: None) -> None: ...
+@overload
+def always_bytes(str_or_bytes: Union[str, bytes]) -> bytes: ...
+def always_bytes(str_or_bytes: Union[None, str, bytes]) -> Optional[bytes]:
+    pass
+
+class Headers:
+    def __init__(self, iter: Iterable[tuple[bytes, bytes]]) -> None: ...
+
+headers: Union[Headers, dict[Union[str, bytes], Union[str, bytes]], Iterable[tuple[bytes, bytes]]]
+
+if isinstance(headers, dict):
+    headers = Headers(
+        (always_bytes(k), always_bytes(v)) for k, v in headers.items()
+    )
+
+reveal_type(headers)  # N: Revealed type is "Union[__main__.Headers, typing.Iterable[tuple[builtins.bytes, builtins.bytes]]]"
+[builtins fixtures/isinstancelist.pyi]

test-data/unit/fixtures/isinstancelist.pyi

@@ -26,6 +26,7 @@ class bool(int): pass
 class str:
     def __add__(self, x: str) -> str: pass
     def __getitem__(self, x: int) -> str: pass
+class bytes: pass
 
 T = TypeVar('T')
 KT = TypeVar('KT')
@@ -52,6 +53,7 @@ class dict(Mapping[KT, VT]):
     def __setitem__(self, k: KT, v: VT) -> None: pass
     def __iter__(self) -> Iterator[KT]: pass
     def update(self, a: Mapping[KT, VT]) -> None: pass
+    def items(self) -> Iterable[Tuple[KT, VT]]: pass
 
 class set(Generic[T]):
     def __iter__(self) -> Iterator[T]: pass