# Augmented assignment

## Basic

```py
x = 3
x -= 1
reveal_type(x)  # revealed: Literal[2]

x = 1.0
x /= 2
reveal_type(x)  # revealed: int | float
```

## Dunder methods

```py
class C:
    def __isub__(self, other: int) -> str:
        return "Hello, world!"

x = C()
x -= 1
reveal_type(x)  # revealed: str

class C:
    def __iadd__(self, other: str) -> int:
        return 1

x = C()
x += "Hello"
reveal_type(x)  # revealed: int
```

## Unsupported types

```py
class C:
    def __isub__(self, other: str) -> int:
        return 42

x = C()
# error: [unsupported-operator] "Operator `-=` is unsupported between objects of type `C` and `Literal[1]`"
x -= 1

reveal_type(x)  # revealed: int
```

## Method union

```py
def _(flag: bool):
    class Foo:
        if flag:
            def __iadd__(self, other: int) -> str:
                return "Hello, world!"
        else:
            def __iadd__(self, other: int) -> int:
                return 42

    f = Foo()
    f += 12

    reveal_type(f)  # revealed: str | int
```

## Partially bound `__iadd__`

```py
def _(flag: bool):
    class Foo:
        if flag:
            def __iadd__(self, other: str) -> int:
                return 42

    f = Foo()

    # error: [unsupported-operator] "Operator `+=` is unsupported between objects of type `Foo` and `Literal["Hello, world!"]`"
    f += "Hello, world!"

    reveal_type(f)  # revealed: int | Unknown
```

## Partially bound with `__add__`

```py
def _(flag: bool):
    class Foo:
        def __add__(self, other: str) -> str:
            return "Hello, world!"
        if flag:
            def __iadd__(self, other: str) -> int:
                return 42

    f = Foo()
    f += "Hello, world!"

    reveal_type(f)  # revealed: int | str
```

## Partially bound target union

```py
def _(flag1: bool, flag2: bool):
    class Foo:
        def __add__(self, other: int) -> str:
            return "Hello, world!"
        if flag1:
            def __iadd__(self, other: int) -> int:
                return 42

    if flag2:
        f = Foo()
    else:
        f = 42.0
    f += 12

    reveal_type(f)  # revealed: int | str | float
```

## Target union

```py
def _(flag: bool):
    class Foo:
        def __iadd__(self, other: int) -> str:
            return "Hello, world!"

    if flag:
        f = Foo()
    else:
        f = 42
    f += 12

    reveal_type(f)  # revealed: str | Literal[54]
```

## Partially bound target union with `__add__`

```py
def f(flag: bool, flag2: bool):
    class Foo:
        def __add__(self, other: int) -> str:
            return "Hello, world!"
        if flag:
            def __iadd__(self, other: int) -> int:
                return 42

    class Bar:
        def __add__(self, other: int) -> bytes:
            return b"Hello, world!"

        def __iadd__(self, other: int) -> float:
            return 42.0

    if flag2:
        f = Foo()
    else:
        f = Bar()
    f += 12

    reveal_type(f)  # revealed: int | str | float
```
