Should unreachable code be considered an error?

[Don]

Sean Kelly wrote:
> On Aug 18, 2011, at 10:29 AM, Nick Sabalausky wrote:
> 
>> "Bernard Helyer" <b.helyer at gmail.com> wrote in message 
>> news:j2ithq$12kd$1 at digitalmars.com...
>>> I asked the Ars forums ( http://arstechnica.com/civis/viewtopic.php?
>>> f=20&t=1153378&p=21965411 ) and I ask the same of you: should
>>> unambiguously unreachable code be an error or a warning? ( see the linked
>>> forum post for more details ).
>> No. That would be a royal pain in the ass during debugging. I expect to be 
>> able to stick a "return xxxx;" anywhere I want to test something and not 
>> have the compiler crap out because I didn't deal with the overhead of 
>> commenting out the rest.
>>
>> A warning might be nice, though.
> 
> A warning if anything.  I've never encountered a situation where code was made unreachable by accident.  I also get "unreachable code" warnings periodically, for code that is absolutely reachable.  I don't want my code to not compile simply because the compiler can't perform adequate flow analysis.

I have encountered bugs of the form:
if (cond) { /* unreachable */ }
and the cond was unintentionally always false. The last time I 
encountered such a bug was last week. I'm surprised your experience is 
so different.

It's crucial that it should never report "unreachable" if it is unsure 
(not even a warning).
But I think conditional compilation is a huge problem -- code may be 
valid under different compilation conditions. I suspect that to 
eliminate all the false positives, it'd have to be so conservative, that 
it wouldn't catch any bugs.