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.
