[1.18 regression] False negative with variable increment in a walrus inside a tuple

[hamdanal]

Bug Report

mypy sometimes fails to report errors in assignment expressions inside a tuple.

To Reproduce

def condition() -> bool: return False

def fn() -> tuple[int, int]:
    i: int | None = 0 if condition() else None
    return (i, (i := i + 1))

print(fn())

Expected Behavior

Something like

lib.py:5: error: Incompatible return value type (got "tuple[int | None, int]", expected "tuple[int, int]")  [return-value]
lib.py:5: error: Unsupported operand types for + ("None" and "int")  [operator]
lib.py:5: note: Left operand is of type "int | None"

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 1.19.1 and master
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.14.2

The weird thing is that if I change assignment target to a different variable like (i, (j := i + 1)) mypy reports the two errors above.
Possibly related to mypyc/mypyc#1179

[hamdanal]

Looks like a regression as this did raise an error in version 1.17.1.

Removed the affects mypyc label as even with version 1.17.1 mypyc has the same problem as in the linked issue.

Bisects to commit a856e55 (#19767)
/cc @ilevkivskyi as the author of the change