Wait.. What? Why would that be the case? And .. why would you state that you'd lose your warranty and prepend 'of course'?
As far as I'm aware you do NOT lose your warranty. That would be rather stupid (of course).
But even if you manage to do that somehow, that is a specific problem. Using a general statement like 'flashing voids your warranty' remains weird (and wrong imo).
"Stuffing your phone in your pockets voids your warranty. Because if you stuff it in your back pocket and sit down on it, hard, it's not the manufacturer's fault if the screen breaks. Is it?"
That said, even if you flash stuff without reading instructions or fully understanding what you do: Most errors I've seen are flat-out not working (software stating: "Can't do this, you're messing up") or recoverable/soft-bricks only.
(In the end it doesn't matter if you CAN kill your phone by flashing nonsense: That still wouldn't make a good argument for "Flashing voids the manufacturer's warranty". I'd just mean that those individuals who bricked their phone .. might not be covered.)
Apple added "been in water detection" to avoid payout on that very activity.
If I flash my phone and later some part of it breaks because it came bad from factory, that is independent of me flashing the phone and should be covered.
GP replied to one of my posts, saying that there IS a real risk of bricking your phone when you flash it (I don't disagree, but consider it unlikely).
By comparing the process with a totally different ~real risk~ (phone -> toilet) I'm trying to drive the point home that it isn't a problem of either flashing your phone or playing some mobile game on that white throne of yours, it is a problem of being careless. Bricking my phone shouldn't be covered by a warranty. Flash or flush doesn't matter.
But trying to imply that the warranty is void because I installed software is overly broad and crazy: If I flash CM today and the phone dies in a month from now due (battery dead, hardware issues w/ screen or power button, antenna issues, pick anything faulty you had with a vanilla/unchanged handset in the past), then CM isn't to blame and my phone has a working warranty (for all I know, for all I care - and anything else seems completely unacceptable in my world).
Of course the manufacturers, who have vested financial insterest, will make you think otherwise.
Of course, this depends on the laws in your jurisdiction.
"What did you do that bricked your phone?"
"I flashed some custom firmware and stopped the flash mid-way"
"But how? All our phones are locked"
"I unlocked it"
"Aha! That is the real culprit!"
Likewise, if you hack special software onto the phone, overriding protections that they put in place to prevent it, I could see them refusing to spend time and money trying to fix the phone until you undid those changes first. And I believe some of those changes can't be un-done fully, even if you re-flash a stock firmware.
The net effect is that it effectively voids your warranty.
So no, technically you don't lose your warranty, but you also can't really make use of it, either.
This really only applies to software things, though. Hardware problems, like the battery exploding or the phone just refusing to turn on would still be easy to get covered, since you can't even get far enough to see it's been modified.
But if the GPS stops working or it stops receiving calls, they could blame it on the custom firmware, and they might even be correct.
If I hand in my handset to claim my warranty, I'm fine with a 'wipe by default' policy. In fact, my limited experiences (both with my phones and the ones from my closest family) that was implied anyway: You'd get the phone back with a ~fresh~ factory image (and all the crap- and adware that this includes).
So here's the thing: If I claim that GPS doesn't work, you reflash it with a Known Good™ image and GPS still doesn't work - then the device has a hardware problem.
If I can cause a hardware problem by running a different flavor of Android, then .. that's a serious hardware _design_ problem.
I'm not saying that you cannot kill or damage a device. But defaulting to 'You installed stuff, you probably caused it' seems rather simplistic, is probably wrong a good number of times.
You brought up PCs: I think you'd have a hard time if you'd defend the stance that installing Linux on a laptop voids the warranty. Granted, the underpaid tech support might not be able to help me around software issues, but if I claim that the wifi doesn't work and send it in... Do you believe they can/could/should refuse to check and fix the problem under warranty?
I'm actually saying that they did refuse to attempt to fix the problem unless you restored the original OS that they sold it to you with. (I believe upgrades to newer versions was okay, because those were supported on newer computers. I didn't encounter that situation in the time I was there.)
Having said that, I didn't always toe the company line. I often did my best to help people who were using an unsupported configuration (added internal hardware, different OS) but there was still a limit to what I could do. Unlike most of the others, though, I had a few years experience working at a computer shop, so I actually knew things and wasn't just reading from the company intranet's guides.
I got caught a couple times on software problems that appeared like hardware problems, too. By not forcing them to revert to the original software, and then escalating them to second tier support, I ended up getting in trouble. So there was at least some method to their madness.
Sending it in, though... There wasn't a place to do that. So far as I know, there weren't any physical stores other than big name retailers like Best Buy, and you couldn't just ship it in without going through phone support. So you had to try the advice from the phone support first before shipping it in.