Or is the "melting point" of polymer just way too high, that no "summer heat in the USA" would ever negatively affect a handgun left sitting in the heat for long periods of time even if polymer?
This. BillM's post is consistent with the research I've done on the polymers typically used in firearms.
You know, actually, I do wonder: how do part POLYMER revolvers/pistols do if they are a "truck gun" left in the summer heat, all summer, 24/7?
They get hot in the summer, they get cold in the winter. They keep working.
Any ammunition should probably be rotated out and used at the range after a couple of seasonal cycles.
A good rule of thumb for polymer firearms. If you can pick them up by the polymer frame without blistering your hand they are not only undamaged, they are also safe to use. At the point where you must don gloves to use them because touching the frame causes injury, it's probably a good idea to let them cool down before trying to shoot them.
Direct UV (outside--not through glass) will break down polymers, but manufacturers are aware of this and the good ones will choose the type of polymer and the specific formulation to minimize this and will also include additives to protect against UV damage. Glass is a UV filter and blocks a lot of UV, so leaving a gun in the car where the sunlight could hit it wouldn't be an issue. Leaving it out on the patio could cause degradation over time.
As I recall, Glock's testing of their particular polymer formulation showed that the frame, if exposed to 100 years of continuous sunlight, would show a 1% mechanical degradation.
I never garage my cars. I had one Honda that sat for 14 years in the TX weather year round. When I sold it, the plastic interior was in perfect condition inside with the exception of places where it had been physically damaged or worn from use. No damage at all visible from the sun or heat.
Frankly, it depends heavily on what kind of a premium the manufacturer puts on longevity. A friend of mine had a car from a different manufacturer than mine and his dash cracked after only a few years under the same conditions as mine.