[Python] Why is mypy --check-untyped-defs not raising trivial error?

The help said:

--check-untyped-defs Type check the interior of functions without type
annotations (inverse: --no-check-untyped-defs)


Why is mypy not able to find this trivial type error?

def show(items):
for item in items:
print(item)

a: int = 10
show(a)

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