My friend is implementing an app for
Amazon Alexa which currently speaks the indefinite article "an" in noun phrase acronyms which start with the letter 'U', for example:
(1.) *I found out he was an U.S. president.
My idiolect prefers "a" here instead, presumably because my aggregated experience of the "a/an" contrast perceived this as a rare (unique?) case where English formally licenses a syntactic alternation (insofar as spelling is overt syntax, and syntax is formal) driven by phonotactic considerations.
So much for the grade-school rule-of-thumb about "whether the spelling starts with a vowel." In any case, I'm a native speaker and it does seem to be the case for me:
(2a.) I'm an unhappy camper. (2b.) Bob is a used car salesman.
Since "U.S." in sentence (1) is pronounced in as in (2b), I prefer the phonologically reduced form:
(3) I found out he was a U.S. president.
Apparently contradicting this idea is the evidence of native speakers who insist on "an" for words starting with 'H'.
(4) ?It was an historic victory.
I marked (4) questionable to indicate that it's not in my idiolect. My question is, without devolving into a prescriptive/descriptive debate, are there other considerations beyond what I've mentioned here that could explain or underlie the observed alternation?