The Slice Scanner's Rules are Incomplete

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When regenerating the Slice scanner (located at cpp/src/Slice/Scanner.l), the following warning is emitted:

src/Slice/Scanner.l:550: warning, -s option given but default rule can be matched

This message means that the scanner is 'incomplete', ie. there is some sequence of characters it will fail to match against. And instead, it will fall back to the 'default rule'. For us, this means a compilation error.

I suspect this was introduced sometime during our backporting from 4.0 to 3.8, but I'm not sure. Unfortunately, Flex doesn't offer any easy to diagnose what the missing case is. But it's existence has no effect on compilation, unless the missing character sequence is hit. So I'm inclined to ignore this, until someone stumbles across the problem.