[PATCH,c++] fix PR 4452{2,3}, autocorrect colon to scopes when appropriate

[Nathan Froyd]

On Thu, Dec 09, 2010 at 04:13:06PM +0100, Dodji Seketeli wrote:
> Nathan Froyd <froydnj@codesourcery.com> writes:
> > @@ -4553,6 +4559,16 @@ cp_parser_nested_name_specifier_opt (cp_parser *parser,
> >  	     template-id), nor a `::', then we are not looking at a
> >  	     nested-name-specifier.  */
> >  	  token = cp_lexer_peek_nth_token (parser->lexer, 2);
> > +
> > +	  if (token->type == CPP_COLON
> > +	      && parser->colon_corrects_to_scope_p
> > +	      && cp_lexer_peek_nth_token (parser->lexer, 3)->type == CPP_NAME)
> > +	    {
> > +	      error_at (token->location,
> > +			"found %<:%> in nested-name-specifier, expected %<::%>");
> > +	      token->type = CPP_SCOPE;
> > +	    }
> > +
> 
> I wonder how this would behave if cp_parser_nested_name_specifier_opt is
> called during a tentative parse that should (silently) fail. In that
> case my understanding is we would issue an error message even though we
> are just parsing tentatively.

That's a good question.  I know that this code gets hit when parsing
tenatively (see the testcase in PR 39859, probably also the testcases I
included with the patch).  I don't know whether there's a case where we
could auto-correct, issue an error, fail, reparse with no
auto-correction, and succeed.  Hopefully there's no cases where that
would happen; I don't think the auto-correct would be very useful if we
limited it to non-tenative parses.

> Also, I am thinking maybe enabling the auto-correcting by default and
> picking where not to enable it can let us allow ':' instead of :: in too
> many places. Perhaps we should rather disable auto-correcting by default
> and just enable it during the parsing of a simple-declaration. Maybe I
> am just being overly conservative :-)

I think the optimistic view (auto-correct always, turn it off when you
can't) is somewhat more feasible; there's only so many places in the
grammar where you can have `:' and only so many places where you'd want
to correct it.

-Nathan