[Bug c++/95111] coroutines use-after-free with lambdas

["ville.voutilainen at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95111

--- Comment #17 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
(In reply to Ville Voutilainen from comment #16)
> (In reply to Iain Sandoe from comment #14)
> > (In reply to Ville Voutilainen from comment #12)
> > The idea of bringing the lambda's captures into the coro frame was what I
> > originally implemented.  Richard pointed out that this is wrong when the
> > lambda is mutable (see
> > gcc/testsuite/g++.dg/coroutines/torture/lambda-10-mutable.C)
> > 
> > so if one has
> > 
> > auto X = [...] () -> some_coro<xxx> {};
> > 
> > X must exist for the duration of the lambda coro [it was pointed out by
> > Lewis that really this is only the same as saying that if you have a class
> > with a member function lambda, the instance of that class has to persist for
> > the duration of the coro].
> 
> Ah. So the work-around for this problem is to copy the capture to a local
> variable, and co_return that; then the local variable is in the coro-state.
> Right?

That is, instead of writing

[x] {co_return x;}

write

[x] {auto xx = x; co_return xx;}