On Mon, Jan 30, 2017 at 4:48 PM, Ville Voutilainen <ville.voutilai...@gmail.com> wrote: > On 31 January 2017 at 00:41, Ville Voutilainen > <ville.voutilai...@gmail.com> wrote: >> On 31 January 2017 at 00:06, Tim Song <t.canens....@gmail.com> wrote: >>> On Mon, Jan 30, 2017 at 9:36 PM Jonathan Wakely <jwak...@redhat.com> wrote: >>>> >>>> On 30/01/17 13:28 +0000, Jonathan Wakely wrote: >>>> >On 30/01/17 13:47 +0200, Ville Voutilainen wrote: >>>> >>Tested on Linux-x64. >>>> > >>>> >OK, thanks. >>>> >>>> To be clear: this isn't approved by LWG yet, but I think we can be a >>>> bit adventurous with deduction guides and add them for experimental >>>> C++17 features. Getting more usage experience before we standardise >>>> these things will be good, and deduction guides are very new and >>>> untried. If we find problems we can remove them again, and will have >>>> invaluable feedback for the standards committee. >>>> >>> >>> My brain compiler says that this may cause problems with >>> >>> std::optional<int> o1; >>> std::optional o2 = o1; // wanted optional<int>, deduced >>> optional<optional<int>> >>> >>> Trunk GCC deduces optional<int>, but I don't think it implements >>> P0512R0 yet, which prefers explicit guides to implicit ones before >>> considering partial ordering. This example is very similar to the >>> example in https://timsong-cpp.github.io/cppwp/over.match.best#1.6. >> >> >> I'll see about constraining the guide tomorrow. > > I don't actually need to constrain it, I could just add a guide like > > template <typename _Tp> optional(optional<_Tp>) -> optional<_Tp>; > > However, I'm not convinced I need to. The preference to an explicit guide is, > at least based > on that paper, a tie-breaker rule. If the copy/move constructors are better > matches than the guide, > those should be picked over a guide. Jason?
Currently G++ first tries deduction directly from the intializer like in a call to a function template; only if that fails does it consider deduction guides. How to handle class deduction WRT implicitly-declared constructors is still very much under discussion. Jason
