Re: Lookahead vs. Scanner Feedback

bliss@sp64.csrd.uiuc.edu (Brian Bliss) writes:


> One place where every yacc/lex based C compiler I know of is
> broken is on a typedef name redefined in an inner scope:
>
> typedef int foo;
>
> main ()
> {
> char foo;
> }
>
> is legal ANSI C. I think there was a thread on this a while back.


I missed this thread, but anyway:


The above shouldn't be a problem, because this is not really an ambiguous
occurrence. You can deal with that by having a production


any_ident : ident | type_ident;


and using any_ident for the identifier in a declarator (and several other
places). This should be possible without introducing any ambiguities.


But for some parts of the C syntax this is not so easy, for labels you
probably have to expand the any_ident production to allow programs like


typef int foo;
main ()
{ foo: ;
}


because otherwise there is a shift-reduce conflict
(reduce type_ident to any_ident for labels, shift for declarations).
[It's not impossible, but it's tricky and messy to get right. -John]
--

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