Fix strict-optional in extending generic TypedDict (#16398)

Fixes #16395

Author: ilevkivskyi

mypy/semanal_typeddict.py

@@ -37,6 +37,7 @@
     has_placeholder,
     require_bool_literal_argument,
 )
+from mypy.state import state
 from mypy.typeanal import check_for_explicit_any, has_any_from_unimported_type
 from mypy.types import (
     TPDICT_NAMES,
@@ -203,7 +204,8 @@ def add_keys_and_types_from_base(
                 any_kind = TypeOfAny.from_error
             base_args = [AnyType(any_kind) for _ in tvars]
 
-        valid_items = self.map_items_to_base(valid_items, tvars, base_args)
+        with state.strict_optional_set(self.options.strict_optional):
+            valid_items = self.map_items_to_base(valid_items, tvars, base_args)
         for key in base_items:
             if key in keys:
                 self.fail(f'Overwriting TypedDict field "{key}" while merging', ctx)

test-data/unit/check-typeddict.test

@@ -3379,3 +3379,20 @@ bar |= d1  # E: Argument 1 to "__ior__" of "TypedDict" has incompatible type "Di
 bar |= d2  # E: Argument 1 to "__ior__" of "TypedDict" has incompatible type "Dict[int, str]"; expected "TypedDict({'key'?: int, 'value'?: str})"
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict-iror.pyi]
+
+[case testGenericTypedDictStrictOptionalExtending]
+from typing import Generic, TypeVar, TypedDict, Optional
+
+T = TypeVar("T")
+class Foo(TypedDict, Generic[T], total=False):
+    a: Optional[str]
+    g: Optional[T]
+
+class Bar(Foo[T], total=False):
+    other: str
+
+b: Bar[int]
+reveal_type(b["a"])  # N: Revealed type is "Union[builtins.str, None]"
+reveal_type(b["g"])  # N: Revealed type is "Union[builtins.int, None]"
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-typeddict.pyi]