Rename Red Knot (#17820)

crates/ty_python_semantic/resources/mdtest/generics/legacy/variables.md

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+# Legacy type variables
+
+The tests in this file focus on how type variables are defined using the legacy notation. Most
+_uses_ of type variables are tested in other files in this directory; we do not duplicate every test
+for both type variable syntaxes.
+
+Unless otherwise specified, all quotations come from the [Generics] section of the typing spec.
+
+## Type variables
+
+### Defining legacy type variables
+
+> Generics can be parameterized by using a factory available in `typing` called `TypeVar`.
+
+This was the only way to create type variables prior to PEP 695/Python 3.12. It is still available
+in newer Python releases.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+reveal_type(type(T))  # revealed: Literal[TypeVar]
+reveal_type(T)  # revealed: typing.TypeVar
+reveal_type(T.__name__)  # revealed: Literal["T"]
+```
+
+### Directly assigned to a variable
+
+> A `TypeVar()` expression must always directly be assigned to a variable (it should not be used as
+> part of a larger expression).
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+# TODO: no error
+# error: [invalid-legacy-type-variable]
+U: TypeVar = TypeVar("U")
+
+# error: [invalid-legacy-type-variable] "A legacy `typing.TypeVar` must be immediately assigned to a variable"
+TestList = list[TypeVar("W")]
+```
+
+### `TypeVar` parameter must match variable name
+
+> The argument to `TypeVar()` must be a string equal to the variable name to which it is assigned.
+
+```py
+from typing import TypeVar
+
+# error: [invalid-legacy-type-variable] "The name of a legacy `typing.TypeVar` (`Q`) must match the name of the variable it is assigned to (`T`)"
+T = TypeVar("Q")
+```
+
+### No redefinition
+
+> Type variables must not be redefined.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+
+# TODO: error
+T = TypeVar("T")
+```
+
+### Type variables with a default
+
+Note that the `__default__` property is only available in Python ≥3.13.
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T", default=int)
+reveal_type(T.__default__)  # revealed: int
+reveal_type(T.__bound__)  # revealed: None
+reveal_type(T.__constraints__)  # revealed: tuple[()]
+
+S = TypeVar("S")
+reveal_type(S.__default__)  # revealed: NoDefault
+```
+
+### Type variables with an upper bound
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T", bound=int)
+reveal_type(T.__bound__)  # revealed: int
+reveal_type(T.__constraints__)  # revealed: tuple[()]
+
+S = TypeVar("S")
+reveal_type(S.__bound__)  # revealed: None
+```
+
+### Type variables with constraints
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T", int, str)
+reveal_type(T.__constraints__)  # revealed: tuple[int, str]
+
+S = TypeVar("S")
+reveal_type(S.__constraints__)  # revealed: tuple[()]
+```
+
+### Cannot have only one constraint
+
+> `TypeVar` supports constraining parametric types to a fixed set of possible types...There should
+> be at least two constraints, if any; specifying a single constraint is disallowed.
+
+```py
+from typing import TypeVar
+
+# TODO: error: [invalid-type-variable-constraints]
+T = TypeVar("T", int)
+```
+
+[generics]: https://typing.python.org/en/latest/spec/generics.html