Reusing variable previously defined with walrus operator triggers used-before-def error

To Reproduce

x = 3
z = (y := str(x)).index(y.lower())

Expected Behavior

Should be no errors -- this code does not error using mypy v0.991 or prior, so this seems to be a new issue with v1.0.0.

Actual Behavior

error: Name "y" is used before definition [used-before-def]

Your Environment

Thanks for reporting! We will include the fix in the 1.0.1 release.

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