stubtest: treat dicts as a subtype of typeddict

mypy/stubtest.py

@@ -946,6 +946,14 @@ def is_subtype_helper(left: mypy.types.Type, right: mypy.types.Type) -> bool:
         # Pretend Literal[0, 1] is a subtype of bool to avoid unhelpful errors.
         return True
 
+    if (
+        isinstance(right, mypy.types.TypedDictType)
+        and isinstance(left, mypy.types.Instance)
+        and left.type.fullname == "builtins.dict"
+    ):
+        # Special case checks against TypedDicts
+        return True
+
     with mypy.state.strict_optional_set(True):
         return mypy.subtypes.is_subtype(left, right)
 

mypy/test/teststubtest.py

@@ -828,6 +828,39 @@ def test_bad_literal(self) -> Iterator[Case]:
             error='WRONG_BOOL_2',
         )
 
+    @collect_cases
+    def test_special_subtype(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            b1: bool
+            b2: bool
+            b3: bool
+            """,
+            runtime="""
+            b1 = 0
+            b2 = 1
+            b3 = 2
+            """,
+            error="b3",
+        )
+        yield Case(
+            stub="""
+            from typing_extensions import TypedDict
+
+            class _Options(TypedDict):
+                a: str
+                b: int
+
+            opt1: _Options
+            opt2: _Options
+            """,
+            runtime="""
+            opt1 = {"some": "stuff"}
+            opt2 = 0
+            """,
+            error="opt2",
+        )
+
 
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!