Re: [C++ Patch] PR 58633

[Jason Merrill <jason at redhat dot com>]

On 10/09/2013 11:14 AM, Mitchell, Mark wrote:
> Unfortunately, I no longer remember this code well.  My recollection --
> such as it is -- is just that I couldn't find a case where it was
> necessary to commit only a single level.  You sometimes had to scan an
> unbounded number of tokens to complete the parse -- but once you
> determined what you had, you knew exactly what had happened up to this
> point.  Perhaps you have now found a counter-example, or perhaps
> something new in the language has changed the situation -- or perhaps I
> just don't remember correctly.

In this case, we're parsing tentatively for the decltype form, and then 
within that parsing tentatively for a pseudo-destructor.  We see that we 
definitely have a pseudo-destructor, but that doesn't mean that the 
decltype expression has the form we want.

> I think that committing was important for two primary reasons:
>
>  1. We should not have irreversible side-effects while in a tentative
>     state.  For example, we shouldn't create a permanent entry in the
>     symbol table, or issue an error message that might not apply if the
>     tentative parse is aborted.  The "tentative" stuff is only in the
>     parser; there's no easy way to remove a function or variable from
>     the symbol table, or undo other pieces of global state in the
>     compiler as a whole.

Right.  In cases where there aren't side-effects, this shouldn't be an 
issue.  We should only use Paolo's new function in such cases; Paolo, 
please add that to the comment.  The patch is OK with that change.

>  2. Committing allows us to flush the buffer of saved tokens, emit any
>     queued error messages, etc.  So, there is a performance benefit in
>     committing.

I think this particular commit was to improve diagnostics.

> I'm sorry not to be able to be of more help.

That was useful, thank you.

Jason