[ty] Initial test suite for TypedDict (#19686)

## Summary

Adds an initial set of tests based on the highest-priority items in
https://github.com/astral-sh/ty/issues/154. This is certainly not yet
exhaustive (required/non-required, `total`, and other things are
missing), but will be useful to measure progress on this feature.

## Test Plan

Checked intended behavior against runtime and other type checkers.

crates/ty_python_semantic/resources/mdtest/typed_dict.md

@@ -1,16 +1,267 @@
 # `TypedDict`
 
-We do not support `TypedDict`s yet. This test mainly exists to make sure that we do not emit any
-errors for the definition of a `TypedDict`.
+A [`TypedDict`] type represents dictionary objects with a specific set of string keys, and with
+specific value types for each valid key. Each string key can be either required or non-required.
+
+## Basic
+
+Here, we define a `TypedDict` using the class-based syntax:
 
 ```py
-from typing_extensions import TypedDict, Required
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+    age: int | None
+```
+
+New inhabitants can be created from dict literals. When accessing keys, the correct types should be
+inferred based on the `TypedDict` definition:
+
+```py
+alice: Person = {"name": "Alice", "age": 30}
+
+# TODO: this should be `str`
+reveal_type(alice["name"])  # revealed: Unknown
+# TODO: this should be `int | None`
+reveal_type(alice["age"])  # revealed: Unknown
+```
+
+Inhabitants can also be created through a constructor call:
+
+```py
+bob = Person(name="Bob", age=25)
+```
+
+Methods that are available on `dict`s are also available on `TypedDict`s:
+
+```py
+bob.update(age=26)
+```
+
+The construction of a `TypedDict` is checked for type correctness:
+
+```py
+# TODO: these should be errors (invalid argument type)
+eve1a: Person = {"name": b"Eve", "age": None}
+eve1b = Person(name=b"Eve", age=None)
+
+# TODO: these should be errors (missing required key)
+eve2a: Person = {"age": 22}
+eve2b = Person(age=22)
+
+# TODO: these should be errors (additional key)
+eve3a: Person = {"name": "Eve", "age": 25, "extra": True}
+eve3b = Person(name="Eve", age=25, extra=True)
+```
+
+Assignments to keys are also validated:
+
+```py
+# TODO: this should be an error
+alice["name"] = None
+```
+
+Assignments to non-existing keys are disallowed:
+
+```py
+# TODO: this should be an error
+alice["extra"] = True
+```
+
+## Structural assignability
+
+Assignability between `TypedDict` types is structural, that is, it is based on the presence of keys
+and their types, rather than the class hierarchy:
+
+```py
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+
+class Employee(TypedDict):
+    name: str
+    employee_id: int
+
+p1: Person = Employee(name="Alice", employee_id=1)
+
+# TODO: this should be an error
+e1: Employee = Person(name="Eve")
+```
+
+All typed dictionaries can be assigned to `Mapping[str, object]`:
+
+```py
+from typing import Mapping, TypedDict
 
 class Person(TypedDict):
     name: str
     age: int | None
 
-alice: Person = {"name": "Alice", "age": 30}
+m: Mapping[str, object] = Person(name="Alice", age=30)
+```
+
+They can *not* be assigned to `dict[str, object]`, as that would allow them to be mutated in unsafe
+ways:
+
+```py
+from typing import TypedDict
+
+def dangerous(d: dict[str, object]) -> None:
+    d["name"] = 1
+
+class Person(TypedDict):
+    name: str
+
+alice: Person = {"name": "Alice"}
+
+# TODO: this should be an invalid-assignment error
+dangerous(alice)
+
+# TODO: this should be `str`
+reveal_type(alice["name"])  # revealed: Unknown
+```
+
+## Types of keys and values
+
+```py
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+    age: int | None
+
+def _(p: Person) -> None:
+    reveal_type(p.keys())  # revealed: @Todo(Support for `TypedDict`)
+    reveal_type(p.values())  # revealed: @Todo(Support for `TypedDict`)
+```
+
+## Unlike normal classes
+
+`TypedDict` types are not like normal classes. The "attributes" can not be accessed. Neither on the
+class itself, nor on inhabitants of the type defined by the class:
+
+```py
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+    age: int | None
+
+# TODO: this should be an error
+Person.name
+
+# TODO: this should be an error
+Person(name="Alice", age=30).name
+```
+
+## Special properties
+
+`TypedDict` class definitions have some special properties that can be used for introspection:
+
+```py
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+    age: int | None
+
+reveal_type(Person.__total__)  # revealed: @Todo(Support for `TypedDict`)
+reveal_type(Person.__required_keys__)  # revealed: @Todo(Support for `TypedDict`)
+reveal_type(Person.__optional_keys__)  # revealed: @Todo(Support for `TypedDict`)
+```
+
+## Subclassing
+
+`TypedDict` types can be subclassed. The subclass can add new keys:
+
+```py
+from typing import TypedDict
+
+class Person(TypedDict):
+    name: str
+
+class Employee(Person):
+    employee_id: int
+
+alice: Employee = {"name": "Alice", "employee_id": 1}
+
+# TODO: this should be an error (missing required key)
+eve: Employee = {"name": "Eve"}
+```
+
+## Generic `TypedDict`
+
+`TypedDict`s can also be generic.
+
+### Legacy generics
+
+```py
+from typing import Generic, TypeVar, TypedDict
+
+T = TypeVar("T")
+
+class TaggedData(TypedDict, Generic[T]):
+    data: T
+    tag: str
+
+p1: TaggedData[int] = {"data": 42, "tag": "number"}
+p2: TaggedData[str] = {"data": "Hello", "tag": "text"}
+
+# TODO: this should be an error (type mismatch)
+p3: TaggedData[int] = {"data": "not a number", "tag": "number"}
+```
+
+### PEP-695 generics
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import TypedDict
+
+class TaggedData[T](TypedDict):
+    data: T
+    tag: str
+
+p1: TaggedData[int] = {"data": 42, "tag": "number"}
+p2: TaggedData[str] = {"data": "Hello", "tag": "text"}
+
+# TODO: this should be an error (type mismatch)
+p3: TaggedData[int] = {"data": "not a number", "tag": "number"}
+```
+
+## Recursive `TypedDict`
+
+`TypedDict`s can also be recursive, allowing for nested structures:
+
+```py
+from __future__ import annotations
+from typing import TypedDict
+
+class Node(TypedDict):
+    name: str
+    parent: Node | None
+
+root: Node = {"name": "root", "parent": None}
+child: Node = {"name": "child", "parent": root}
+grandchild: Node = {"name": "grandchild", "parent": child}
+
+nested: Node = {"name": "n1", "parent": {"name": "n2", "parent": {"name": "n3", "parent": None}}}
+
+# TODO: this should be an error (invalid type for `name` in innermost node)
+nested_invalid: Node = {"name": "n1", "parent": {"name": "n2", "parent": {"name": 3, "parent": None}}}
+```
+
+## Function/assignment syntax
+
+This is not yet supported. Make sure that we do not emit false positives for this syntax:
+
+```py
+from typing_extensions import TypedDict, Required
 
 # Alternative syntax
 Message = TypedDict("Message", {"id": Required[int], "content": str}, total=False)
@@ -20,6 +271,7 @@ msg = Message(id=1, content="Hello")
 # No errors for yet-unsupported features (`closed`):
 OtherMessage = TypedDict("OtherMessage", {"id": int, "content": str}, closed=True)
 
-reveal_type(Person.__required_keys__)  # revealed: @Todo(Support for `TypedDict`)
 reveal_type(Message.__required_keys__)  # revealed: @Todo(Support for `TypedDict`)
 ```
+
+[`typeddict`]: https://typing.python.org/en/latest/spec/typeddict.html