# PEP 695 type aliases

PEP 695 type aliases are only available in Python 3.12 and later:

```toml
[environment]
python-version = "3.12"
```

## Basic

```py
type IntOrStr = int | str

reveal_type(IntOrStr)  # revealed: typing.TypeAliasType
reveal_type(IntOrStr.__name__)  # revealed: Literal["IntOrStr"]

x: IntOrStr = 1

reveal_type(x)  # revealed: Literal[1]

def f() -> None:
    reveal_type(x)  # revealed: int | str
```

## `__value__` attribute

```py
type IntOrStr = int | str

reveal_type(IntOrStr.__value__)  # revealed: @Todo(Support for `typing.TypeAlias`)
```

## Invalid assignment

```py
type OptionalInt = int | None

# error: [invalid-assignment]
x: OptionalInt = "1"
```

## Type aliases in type aliases

```py
type IntOrStr = int | str
type IntOrStrOrBytes = IntOrStr | bytes

x: IntOrStrOrBytes = 1

def f() -> None:
    reveal_type(x)  # revealed: int | str | bytes
```

## Aliased type aliases

```py
type IntOrStr = int | str
MyIntOrStr = IntOrStr

x: MyIntOrStr = 1

# error: [invalid-assignment]
y: MyIntOrStr = None
```

## Unpacking from a type alias

```py
type T = tuple[int, str]

def f(x: T):
    a, b = x
    reveal_type(a)  # revealed: int
    reveal_type(b)  # revealed: str
```

## Scoping

PEP 695 type aliases delay runtime evaluation of their right-hand side, so they are a lazy (not
eager) nested scope.

```py
type Alias = Foo | str

def f(x: Alias):
    reveal_type(x)  # revealed: Foo | str

class Foo:
    pass
```

But narrowing of names used in the type alias is still respected:

```py
def _(flag: bool):
    t = int if flag else None
    if t is not None:
        type Alias = t | str
        def f(x: Alias):
            reveal_type(x)  # revealed: int | str
```

## Generic type aliases

```py
type ListOrSet[T] = list[T] | set[T]
reveal_type(ListOrSet.__type_params__)  # revealed: tuple[TypeVar | ParamSpec | TypeVarTuple, ...]
```

## In unions and intersections

We can "break apart" a type alias by e.g. adding it to a union:

```py
type IntOrStr = int | str

def f(x: IntOrStr, y: str | bytes):
    z = x or y
    reveal_type(z)  # revealed: (int & ~AlwaysFalsy) | str | bytes
```

## `TypeAliasType` properties

Two `TypeAliasType`s are distinct and disjoint, even if they refer to the same type

```py
from ty_extensions import static_assert, is_equivalent_to, is_disjoint_from, TypeOf

type Alias1 = int
type Alias2 = int

type TypeAliasType1 = TypeOf[Alias1]
type TypeAliasType2 = TypeOf[Alias2]

static_assert(not is_equivalent_to(TypeAliasType1, TypeAliasType2))
static_assert(is_disjoint_from(TypeAliasType1, TypeAliasType2))
```

## Direct use of `TypeAliasType`

`TypeAliasType` can also be used directly. This is useful for versions of Python prior to 3.12.

```toml
[environment]
python-version = "3.9"
```

### Basic example

```py
from typing_extensions import TypeAliasType, Union

IntOrStr = TypeAliasType("IntOrStr", Union[int, str])

reveal_type(IntOrStr)  # revealed: typing.TypeAliasType

reveal_type(IntOrStr.__name__)  # revealed: Literal["IntOrStr"]

def f(x: IntOrStr) -> None:
    reveal_type(x)  # revealed: int | str
```

### Generic example

```py
from typing_extensions import TypeAliasType, TypeVar

T = TypeVar("T")

IntAnd = TypeAliasType("IntAndT", tuple[int, T], type_params=(T,))

def f(x: IntAnd[str]) -> None:
    reveal_type(x)  # revealed: @Todo(Generic PEP-695 type alias)
```

### Error cases

#### Name is not a string literal

```py
from typing_extensions import TypeAliasType

def get_name() -> str:
    return "IntOrStr"

# error: [invalid-type-alias-type] "The name of a `typing.TypeAlias` must be a string literal"
IntOrStr = TypeAliasType(get_name(), int | str)
```

## Cyclic aliases

### Self-referential

```py
type OptNestedInt = int | tuple[OptNestedInt, ...] | None

def f(x: OptNestedInt) -> None:
    reveal_type(x)  # revealed: int | tuple[OptNestedInt, ...] | None
    if x is not None:
        reveal_type(x)  # revealed: int | tuple[OptNestedInt, ...]
```

### Invalid self-referential

```py
# TODO emit a diagnostic on these two lines
type IntOr = int | IntOr
type OrInt = OrInt | int

def f(x: IntOr, y: OrInt):
    reveal_type(x)  # revealed: int
    reveal_type(y)  # revealed: int
    if not isinstance(x, int):
        reveal_type(x)  # revealed: Never
    if not isinstance(y, int):
        reveal_type(y)  # revealed: Never
```

### Mutually recursive

```py
type A = tuple[B] | None
type B = tuple[A] | None

def f(x: A):
    if x is not None:
        reveal_type(x)  # revealed: tuple[B]
        y = x[0]
        if y is not None:
            reveal_type(y)  # revealed: tuple[A]

def g(x: A | B):
    reveal_type(x)  # revealed: tuple[B] | None

from ty_extensions import Intersection

def h(x: Intersection[A, B]):
    reveal_type(x)  # revealed: tuple[B] | None
```

### Union inside generic

#### With old-style union

```py
from typing import Union

type A = list[Union["A", str]]

def f(x: A):
    reveal_type(x)  # revealed: list[A | str]
    for item in x:
        reveal_type(item)  # revealed: list[A | str] | str
```

#### With new-style union

```py
type A = list["A" | str]

def f(x: A):
    reveal_type(x)  # revealed: list[A | str]
    for item in x:
        reveal_type(item)  # revealed: list[A | str] | str
```

#### With Optional

```py
from typing import Optional, Union

type A = list[Optional[Union["A", str]]]

def f(x: A):
    reveal_type(x)  # revealed: list[A | str | None]
    for item in x:
        reveal_type(item)  # revealed: list[A | str | None] | str | None
```
