Re: [PATCH] PR libstdc++/80553 don't allow destroying non-destructible types

[Jonathan Wakely <jwakely at redhat dot com>]

On 02/05/17 17:30 +0200, Marc Glisse wrote:
> On Tue, 2 May 2017, Jonathan Wakely wrote:
>
> > On 02/05/17 10:16 +0100, Jonathan Wakely wrote:
> > > On 28/04/17 13:56 +0100, Jonathan Wakely wrote:
> > > > We optimize _Destroy and _Destroy_n to do nothing when the type has a
> > > > trivial destructor, which means we do nothing (instead of giving an
> > > > error) when trying to destroy types with deleted destructors.
> > >
> > > I wonder if this optimisation should even exist. The compiler should
> > > be able to optimise away a loop that just calls trivial destructors,
> > > without help from the library.
> >
> > The compiler can indeed do that optimisation, even for destructors
> > like ~T() { } that are empty, but not trivial according to the
> > language rules. The libstdc++ optimisation does make a difference at
> > -O0 though. If we get any more bugs in that code I think we should
> > just remove it though, and let the compiler do the right thing.
>
> Does the compiler manage it for all containers, even those with 
> iterators much more complicated than vector's?

It seems to for std::deque (not very complicated) and std::map
(moderately complicated). I didn't try for something like a
directory_iterator which almost certainly wouldn't get optimised away!

> I'd rather keep the 
> special code in the library, if it doesn't cause too much trouble.

Yes, assuming the code's correct now then we might as well keep it,
but if it's a source of too many more bugs then it starts to cause too
much trouble.