New macro expander preamble

[Neil Booth]

Zack Weinberg wrote:-

> > The double warning is caused because parse_identifier handles the '?'
> > (it might be an escaped newline) as well as _cpp_lex_token for the
> > next token; in other words it's handled twice.  It's not restricted
> > to stdin - any file works too, so we can put it in the testsuite.
> > 
> > I can't see a clean way to fix this just yet; but it's on my list.
> 
> Maybe have parse_identifier emit the warning only for ??/ and only
> when converted?  Otherwise, we know _cpp_lex_token will get it.  This
> might speed up the exit path from p_i, too.

Zack,

I've had a good look at this bug; surprisingly it's quite subtle.  I
can't see an easy way to get it right in all cases - identifiers,
numbers, second character of a token etc. etc.

This is low priority, but it is on my list.  I'll come back to it
later.

Neil.