`if x is None: x = <unnanotated call>` doesn't clear `None`

[MarcoGorelli]

Describe the Bug

def untyped_fallback(x):  # deliberately unannotated
    return x


def f(x: str | None) -> str:
    if x is None:
        y = untyped_fallback("default")
        print(y.upper())
        x = y
        print(x.upper())
    return x.upper()

ERROR sandbox.py:10:15-22: Object of class `NoneType` has no attribute `upper` [[missing-attribute](https://pyrefly.org/en/docs/error-kinds/#missing-attribute)]
ERROR sandbox.py:11:12-19: Object of class `NoneType` has no attribute `upper` [[missing-attribute](https://pyrefly.org/en/docs/error-kinds/#missing-attribute)]

Here, y is Unknown, as untyped_fallback. So y.upper() isn't flagged. What's surprising to me is that then, after x=y, x.upper() is flagged, saying that None doesn't have an upper attribute

No other type checker I tried reports on this

Spotted in Hypothesis

Sandbox Link

https://pyrefly.org/sandbox/?project=v2.pZRNDoIwEIWv0sgCXEj3nMIDkJgKFY2kEKCmvb1vBhr5UTYu275OpvO-vl-RSRZaQ5lSXm54-VUVz8QdMyEi2IvPSaGhaw-RooaxQATMsHHk1AR04vhjTggzFFh-wHVfoPV47qaBnPpSth7yAywO0tF3n9q21V1ynJ04FPFroVsJQ8Nhe2tsJM7jhphmhEgDEZXFDOB0mpsImanFfRjaPpMyXG-6Smojy6bo5eKGFMSweqkHx_GynGgYxB5l_yEqHoGK93iKCSdWnJTxEPEksM_FcbDLzhs

(Only applicable for extension issues) IDE Information

No response