Variable assigned in branches of an if statement is considered as being redefined

Bug Report

The same variable assigned in different branches of an if statement is considered as being redefined, despite the fact that at in reality there is only one assignment being performed, it's just unknown at that stage which branch will be taken.

To Reproduce

from io import BytesIO
from typing import ClassVar

class IntegerAdapter:
    _size_: ClassVar[int] = 4

    @classmethod
    def from_wire(cls, buffer: bytes | bytearray | memoryview | BytesIO) -> int:
        if isinstance(buffer, BytesIO):
            data = buffer.read(cls._size_)
        else:
            data = buffer[:cls._size_]
        return int.from_bytes(data, byteorder='big', signed=True)

Expected Behavior

I would expect mypy to infer the type of the variable as the union of the types in all branches. This is not really a redefinition since the variable is not assigned twice in succession, only one branch is taken, it's just unknown which one.
Using --allow-redefinition doesn't help as one of the requirements for that is to read the variable before the next assignment, which is impossible.

Also the behavior is inconsistent with different ways of writing the if statement. Inverting the condition and reversing the branches makes the error go away. So does using an equivalent conditional expression.

Any of the following ways to rewrite the if statement will eliminate the error, but one should not have to retort to such gimmicks to avoid this.

        # this works because the 1st assignment is the one with a wider type that includes the type of the 2nd
        if not isinstance(buffer, BytesIO):
            data = buffer[:cls._size_]
        else:
            data = buffer.read(cls._size)

        # this works because the type is correctly inferred to be the union of the branch types
        data = buffer.read(cls._size_) if isinstance(buffer, BytesIO) else buffer[:cls._size_]

IMO the conditional statement is the only one that behaves correctly here as it infers the type as the union of the two.
But semantically the if-else and the conditional statement are the same, just differently written: take one branch and assign computed value to the variable.

Actual Behavior

typing-6.py:14: error: Incompatible types in assignment (expression has type "bytes | bytearray | memoryview", variable has type "bytes")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.12.0+dev.6a0657e5959ba1777c4d427f8f355d499035d145 (compiled: no)
• Mypy command-line flags: with or without --allow-redefinition, makes no difference
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
enable_incomplete_feature = "NewGenericSyntax"
disable_bytearray_promotion = true
disable_memoryview_promotion = true
check_untyped_defs = true
warn_unreachable = true
warn_redundant_casts = true
warn_unused_ignores = true

• Python version used: 3.12

首先使用 mypy 和所示配置运行提供的 Python reproducer，并比较 if-else 形式与等价的条件表达式。跟踪分支赋值的检查方式，并让 if-else 情况推断出 union，而不报告不兼容的重新定义；验证 reproducer 在没有错误的情况下完成。