# Generic functions: PEP 695 syntax

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

## Typevar must be used at least twice

If you're only using a typevar for a single parameter, you don't need the typevar — just use
`object` (or the typevar's upper bound):

```py
# TODO: error, should be (x: object)
def typevar_not_needed[T](x: T) -> None:
    pass

# TODO: error, should be (x: int)
def bounded_typevar_not_needed[T: int](x: T) -> None:
    pass
```

Typevars are only needed if you use them more than once. For instance, to specify that two
parameters must both have the same type:

```py
def two_params[T](x: T, y: T) -> T:
    return x
```

or to specify that a return value is the same as a parameter:

```py
def return_value[T](x: T) -> T:
    return x
```

Each typevar must also appear _somewhere_ in the parameter list:

```py
def absurd[T]() -> T:
    # There's no way to construct a T!
    raise ValueError("absurd")
```

## Inferring generic function parameter types

If the type of a generic function parameter is a typevar, then we can infer what type that typevar
is bound to at each call site.

```py
def f[T](x: T) -> T:
    return x

reveal_type(f(1))  # revealed: Literal[1]
reveal_type(f(1.0))  # revealed: float
reveal_type(f(True))  # revealed: Literal[True]
reveal_type(f("string"))  # revealed: Literal["string"]
```

## Inferring “deep” generic parameter types

The matching up of call arguments and discovery of constraints on typevars can be a recursive
process for arbitrarily-nested generic classes and protocols in parameters.

TODO: Note that we can currently only infer a specialization for a generic protocol when the
argument _explicitly_ implements the protocol by listing it as a base class.

```py
from typing import Protocol, TypeVar

S = TypeVar("S")

class CanIndex(Protocol[S]):
    def __getitem__(self, index: int) -> S: ...

class ExplicitlyImplements[T](CanIndex[T]): ...

def takes_in_list[T](x: list[T]) -> list[T]:
    return x

def takes_in_protocol[T](x: CanIndex[T]) -> T:
    return x[0]

def deep_list(x: list[str]) -> None:
    reveal_type(takes_in_list(x))  # revealed: list[str]
    # TODO: revealed: str
    reveal_type(takes_in_protocol(x))  # revealed: Unknown

def deeper_list(x: list[set[str]]) -> None:
    reveal_type(takes_in_list(x))  # revealed: list[set[str]]
    # TODO: revealed: set[str]
    reveal_type(takes_in_protocol(x))  # revealed: Unknown

def deep_explicit(x: ExplicitlyImplements[str]) -> None:
    reveal_type(takes_in_protocol(x))  # revealed: str

def deeper_explicit(x: ExplicitlyImplements[set[str]]) -> None:
    reveal_type(takes_in_protocol(x))  # revealed: set[str]

def takes_in_type[T](x: type[T]) -> type[T]:
    return x

reveal_type(takes_in_type(int))  # revealed: @Todo(unsupported type[X] special form)
```

This also works when passing in arguments that are subclasses of the parameter type.

```py
class Sub(list[int]): ...
class GenericSub[T](list[T]): ...

reveal_type(takes_in_list(Sub()))  # revealed: list[int]
# TODO: revealed: int
reveal_type(takes_in_protocol(Sub()))  # revealed: Unknown

reveal_type(takes_in_list(GenericSub[str]()))  # revealed: list[str]
# TODO: revealed: str
reveal_type(takes_in_protocol(GenericSub[str]()))  # revealed: Unknown

class ExplicitSub(ExplicitlyImplements[int]): ...
class ExplicitGenericSub[T](ExplicitlyImplements[T]): ...

reveal_type(takes_in_protocol(ExplicitSub()))  # revealed: int
reveal_type(takes_in_protocol(ExplicitGenericSub[str]()))  # revealed: str
```

## Inferring tuple parameter types

```py
def takes_mixed_tuple_suffix[T](x: tuple[int, bytes, *tuple[str, ...], T, int]) -> T:
    return x[-2]

# TODO: revealed: Literal[True]
reveal_type(takes_mixed_tuple_suffix((1, b"foo", "bar", "baz", True, 42)))  # revealed: Unknown

def takes_mixed_tuple_prefix[T](x: tuple[int, T, *tuple[str, ...], bool, int]) -> T:
    return x[1]

# TODO: revealed: Literal[b"foo"]
reveal_type(takes_mixed_tuple_prefix((1, b"foo", "bar", "baz", True, 42)))  # revealed: Unknown

def takes_fixed_tuple[T](x: tuple[T, int]) -> T:
    return x[0]

reveal_type(takes_fixed_tuple((True, 42)))  # revealed: Literal[True]

def takes_homogeneous_tuple[T](x: tuple[T, ...]) -> T:
    return x[0]

# TODO: revealed: Literal[42]
reveal_type(takes_homogeneous_tuple((42,)))  # revealed: Unknown
# TODO: revealed: Literal[42, 43]
reveal_type(takes_homogeneous_tuple((42, 43)))  # revealed: Unknown
```

## Inferring a bound typevar

<!-- snapshot-diagnostics -->

```py
from typing_extensions import reveal_type

def f[T: int](x: T) -> T:
    return x

reveal_type(f(1))  # revealed: Literal[1]
reveal_type(f(True))  # revealed: Literal[True]
# error: [invalid-argument-type]
reveal_type(f("string"))  # revealed: Unknown
```

## Inferring a constrained typevar

<!-- snapshot-diagnostics -->

```py
from typing_extensions import reveal_type

def f[T: (int, None)](x: T) -> T:
    return x

reveal_type(f(1))  # revealed: int
reveal_type(f(True))  # revealed: int
reveal_type(f(None))  # revealed: None
# error: [invalid-argument-type]
reveal_type(f("string"))  # revealed: Unknown
```

## Typevar constraints

If a type parameter has an upper bound, that upper bound constrains which types can be used for that
typevar. This effectively adds the upper bound as an intersection to every appearance of the typevar
in the function.

```py
def good_param[T: int](x: T) -> None:
    reveal_type(x)  # revealed: T
```

If the function is annotated as returning the typevar, this means that the upper bound is _not_
assignable to that typevar, since return types are contravariant. In `bad`, we can infer that
`x + 1` has type `int`. But `T` might be instantiated with a narrower type than `int`, and so the
return value is not guaranteed to be compatible for all `T: int`.

```py
def good_return[T: int](x: T) -> T:
    return x

def bad_return[T: int](x: T) -> T:
    # error: [invalid-return-type] "Return type does not match returned value: expected `T`, found `int`"
    return x + 1
```

## All occurrences of the same typevar have the same type

If a typevar appears multiple times in a function signature, all occurrences have the same type.

```py
def different_types[T, S](cond: bool, t: T, s: S) -> T:
    if cond:
        return t
    else:
        # error: [invalid-return-type] "Return type does not match returned value: expected `T`, found `S`"
        return s

def same_types[T](cond: bool, t1: T, t2: T) -> T:
    if cond:
        return t1
    else:
        return t2
```

## All occurrences of the same constrained typevar have the same type

The above is true even when the typevars are constrained. Here, both `int` and `str` have `__add__`
methods that are compatible with the return type, so the `return` expression is always well-typed:

```py
def same_constrained_types[T: (int, str)](t1: T, t2: T) -> T:
    # TODO: no error
    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `T` and `T`"
    return t1 + t2
```

This is _not_ the same as a union type, because of this additional constraint that the two
occurrences have the same type. In `unions_are_different`, `t1` and `t2` might have different types,
and an `int` and a `str` cannot be added together:

```py
def unions_are_different(t1: int | str, t2: int | str) -> int | str:
    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `int | str` and `int | str`"
    return t1 + t2
```

## Typevar inference is a unification problem

When inferring typevar assignments in a generic function call, we cannot simply solve constraints
eagerly for each parameter in turn. We must solve a unification problem involving all of the
parameters simultaneously.

```py
def two_params[T](x: T, y: T) -> T:
    return x

reveal_type(two_params("a", "b"))  # revealed: Literal["a", "b"]
reveal_type(two_params("a", 1))  # revealed: Literal["a", 1]
```

When one of the parameters is a union, we attempt to find the smallest specialization that satisfies
all of the constraints.

```py
def union_param[T](x: T | None) -> T:
    if x is None:
        raise ValueError
    return x

reveal_type(union_param("a"))  # revealed: Literal["a"]
reveal_type(union_param(1))  # revealed: Literal[1]
reveal_type(union_param(None))  # revealed: Unknown
```

```py
def union_and_nonunion_params[T](x: T | int, y: T) -> T:
    return y

reveal_type(union_and_nonunion_params(1, "a"))  # revealed: Literal["a"]
reveal_type(union_and_nonunion_params("a", "a"))  # revealed: Literal["a"]
reveal_type(union_and_nonunion_params(1, 1))  # revealed: Literal[1]
reveal_type(union_and_nonunion_params(3, 1))  # revealed: Literal[1]
reveal_type(union_and_nonunion_params("a", 1))  # revealed: Literal["a", 1]
```

```py
def tuple_param[T, S](x: T | S, y: tuple[T, S]) -> tuple[T, S]:
    return y

reveal_type(tuple_param("a", ("a", 1)))  # revealed: tuple[Literal["a"], Literal[1]]
reveal_type(tuple_param(1, ("a", 1)))  # revealed: tuple[Literal["a"], Literal[1]]
```

## Inferring nested generic function calls

We can infer type assignments in nested calls to multiple generic functions. If they use the same
type variable, we do not confuse the two; `T@f` and `T@g` have separate types in each example below.

```py
def f[T](x: T) -> tuple[T, int]:
    return (x, 1)

def g[T](x: T) -> T | None:
    return x

reveal_type(f(g("a")))  # revealed: tuple[Literal["a"] | None, int]
reveal_type(g(f("a")))  # revealed: tuple[Literal["a"], int] | None
```

## Protocols as TypeVar bounds

Protocol types can be used as TypeVar bounds, just like nominal types.

```py
from typing import Any, Protocol
from ty_extensions import static_assert, is_assignable_to

class SupportsClose(Protocol):
    def close(self) -> None: ...

class ClosableFullyStaticProtocol(Protocol):
    x: int
    def close(self) -> None: ...

class ClosableNonFullyStaticProtocol(Protocol):
    x: Any
    def close(self) -> None: ...

class ClosableFullyStaticNominal:
    x: int
    def close(self) -> None: ...

class ClosableNonFullyStaticNominal:
    x: int
    def close(self) -> None: ...

class NotClosableProtocol(Protocol): ...
class NotClosableNominal: ...

def close_and_return[T: SupportsClose](x: T) -> T:
    x.close()
    return x

def f(
    a: SupportsClose,
    b: ClosableFullyStaticProtocol,
    c: ClosableNonFullyStaticProtocol,
    d: ClosableFullyStaticNominal,
    e: ClosableNonFullyStaticNominal,
    f: NotClosableProtocol,
    g: NotClosableNominal,
):
    reveal_type(close_and_return(a))  # revealed: SupportsClose
    reveal_type(close_and_return(b))  # revealed: ClosableFullyStaticProtocol
    reveal_type(close_and_return(c))  # revealed: ClosableNonFullyStaticProtocol
    reveal_type(close_and_return(d))  # revealed: ClosableFullyStaticNominal
    reveal_type(close_and_return(e))  # revealed: ClosableNonFullyStaticNominal

    # error: [invalid-argument-type] "does not satisfy upper bound"
    reveal_type(close_and_return(f))  # revealed: Unknown
    # error: [invalid-argument-type] "does not satisfy upper bound"
    reveal_type(close_and_return(g))  # revealed: Unknown
```
