Unused variables, better as error or warning?

[Walter Bright]

bearophile wrote:
> If
> your language (C, D, etc) allows you to not use a variable the last time you
> have initialized it, and then you want to add a warning that finds such
> situations, you don't need the compiler to be 100% accurate, even if some of
> such situations are not detected then it's perfectly OK.

Then you're faced with one of two unpleasant consequences:

1. You have to build the logic of what cases you detect into the specification. 
This leads to user confusion about the why's of this and gives a bad impression 
about the arbitrary nature of it.

2. You are faced with errors building with one compiler and no errors on 
another. I.e. your code loses portability.