It can get surprisingly hot in a locked car in direct sun. Not that hot by any means, but it can get well above boiling. It can easily get 90˚ plus (Celsius) in a car in around 35 degree outside temperature. That in itself isn't good for the laptop.
I was just using the worst case scenario. At 130˚, the battery would pretty much explode. I'm guessing the OP is from Hong Kong (based on username) and I'm not sure how hot it gets there.
I lived and worked in Shenzhen for several years, and i would not take the risk on a clear day the sunlight is too strong and the temperature inside the car may at very least affect the batteries runtime at worst it may explode.
I saw first hand the result of a cell phone exploding due to being left on the dash of a Toyota LandCruiser in Saudi, it literally destroyed the interior of the vehicle, spot of explaining to the boss eh In all seriousness a MBP has a lot more battery than a cell phone, I certainly don't leave mine in the passenger compartment here in Malaysia, for a limited time the boot/trunk should be ok as the temperature wont be amplified by the sunlight.
if once in a while, I wouldn't lose too much sleep over. Long term regular, I say don't.
I notice car stereos degrade over time. Gonna blame on vibration and temp extremes.
That's for the electronics.
LCD screen - they don't like heat.
Plastics - Oy, certain trims may warp.
I once left a DDM (digital voltmeter) in the trunk, so OK it was a made-in-China cheap plastic variety and when I returned the thing melted like in certain cartoons! and this is *only* California, not Arizona.