Quuxplusone added a comment.
> - using CTAD on arbitrary class templates that weren't designed for it
> creates a source compatibility problem that the class template author has no
> control over nor say in
> - using CTAD on class templates that were designed for it does not create a
> new source compatibility problem -- the class template author created the
> source compatibility problem themselves when they designed how their class
> templates would interact with CTAD
I don't see the distinction you're making. It sounds like you're saying,
"`std::vector` is a 'CTAD-enabled' type. You (the client programmer) opted into
CTAD when you started using `std::vector` in your project. If you don't want
accidental CTAD, stop using `std::vector`" — which I don't think is practically
useful advice.
> it would also seem like a good idea to have warnings for CTAD cases that we
> have strong evidence are dangerous (eg, for `T(x...)` where `x` expands to a
> pack containing a single `T`, we should warn if we use the copy-deduction
> candidate and there's another candidate that would have been selected if the
> copy-deduction candidate didn't get a priority boost)
That sounds like a good idea (although I am not qualified to implement it). I
would also ask for a warning on `T t{x};` within a generic algorithm where `x`
is of some dependent type that might or might not happen to be a specialization
of `T`. And ideally we'd be able to produce the warning at parse time (i.e.
generic-algorithm-testing time) rather than waiting until the offending
template instantiation is actually instantiated (i.e.
generic-algorithm-usage-in-the-field time), because by then it's too late for
the client programmer to do anything about it.
For an (artificial) example of how accidental CTAD in a generic algorithm can
cause interesting misbehavior, see
https://quuxplusone.github.io/blog/2018/12/11/dont-inherit-from-std-types/#now-we-feed-them-to-this-functio
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D54565/new/
https://reviews.llvm.org/D54565
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
