[Bug c++/19199] [3.3/3.4/4.0/4.1 Regression] Wrong warning about returning a reference to a temporary

[Mark Mitchell]

Roger Sayle wrote:
> On Mon, 7 Mar 2005, Mark Mitchell wrote:
> 
>>Roger Sayle wrote:
>>
>>>I truly hope you're not trying to suggest that it was me that introduced
>>>the concept of MIN_EXPR and MAX_EXPR as lvalues into the C++ front-end:
>>
>>I thought you were the person who introduced changes to fold that caused
>>it to generate these expressions when the GNU source extension did not
>>appear.  Anyhow, who did what to whom isn't important. :-)
> 
> I do appreciate the smiley, but I still feel unfairly maligned by such
> accusations.  

Am I incorrect about the fact that you introduced the changes to fold? 
If so, I humbly retract all previous maligning, and acknowledge the 
unfairness thereof.  If not, then that change probably introduced this 
regression.  But, I'm still not meaning to malign; until I've introduced 
zero regressions in my last thousand patches, who am I to talk?

If I seem frustruated, it's simply because we seem to be running into 
this issue again, after attempting to fix it before.  That's my fault; I 
wasn't comfortable with the fix you came up with for 7503, but instead 
of pushing on that issue, I went along with build_modify_expr change to 
reconsitute COND_EXPRs after the fact.

The way I think about this is that G++ has long supported the GNU 
min/max expression extension -- and it's long been broken.  Over the 
years, I've fielded several bug reports about that extension, and we've 
gradually cleaned it up, but mostly it's just been neglected.  I think 
that you probably got fooled by the fact that the code is there into 
thinking that it works. :-)  (I wouldn't be surprised if it works well 
today, but I'd also not be surprised if it doesn't work.  It certainly 
didn't work reliably at the time of the original CVS import.)

I think that the simplest, most reliable, short-term change is to 
disable this particular optimization in fold, only for C++.  That's not 
an ideal solution.  If you can come up with a better approach that 
doesn't involve disabling the optimization, that's great -- but I think 
we need to convince ourselves that it handles all the cases.  (I 
actually think that your MIN_EXPR-is-a-canonical-form-of-COND_EXPR 
argument can be made to work -- but you need to push the 
build_modify_expr logic into more places.)  In the long term, moving 
fold until after the front end has done its thing, is the right way to go.

-- 
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304