I don't think the following static type checks without a static typing extension:

declare function local:func() as xs:integer
{
text {"1"}
};

text {"1"} uses the following function

fs:item-sequence-to-untypedAtomic-text($items as item()*) as xs:untypedAtomic?

with the static typing rules for state:

"There are no special static typing rules for this function. Static type analysis for this function should be performed as for a built-in function declared with the given signature."

so when the body is type checked it gets an optional quantifier.

Changing the return type to "as xs:integer?" should fix this.