On Thu, 1 Dec 2022 at 23:27, leon zadorin via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > > On Thu, Dec 1, 2022 at 8:35 PM LIU Hao <lh_mouse@xxxxxxx> wrote: > > > > > I think this is a bug in Clang. Instantiation of `shared_ptr<T<U>>` is not > > meant to instantiate > > `T<U>`. Your code actually compiles fine if the definition of `J` was not > > provided. > > > > -- > > Best regards, > > LIU Hao > > > > Thanks :) It is rather interesting, there is a discussion at > https://github.com/llvm/llvm-project/issues/59292 > which may be pointing to implementation differences between stdlibc++ and > libc++ etc. > > There is thinknig that it just may be UB in the originally-posted code (if > so then clang is rather helpful there, but only when used with libstdc++ > apparently.. as I haven't personally verified the delta between building > against different libs :) :) ) > > The current thought prorcess is that std component must be allowed by the > standard to be instantiated with incomplete type T. And vector is allowed > so indeed (but I think it must be complete by the time its members, e.g. > via member-function invocation, are referenced)... so then it may come down > to std::optional and its implementation -- whether its ctor is referencing > any vector's members (dtor etc.) ... > > I suppose the official way would be to find whether ::std::optional may be > instantiated with incomplete type T It may not be. (say even if only when using default, > no value present, ctor overload)... and if it not mentioned of being able > to do so then it would be a bug in the original code (at which point having > some toolchain being able to detect it is rather super nice :) :) > > I dont know to be honest if standard's doco for such ctor , e.g. > constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; > Postconditions: *this does not contain a value. > <https://timsong-cpp.github.io/cppwp/n4861/optional#ctor-1.sentence-1> > Remarks: No contained value is initialized. > <https://timsong-cpp.github.io/cppwp/n4861/optional#ctor-2.sentence-1> > For every object type T these constructors are constexpr > constructors > > would be sufficient to infer the ability to instantiate with incomplete > type... probably not (I'm not enough of an expert to see through this with > my cloudy head atm :) Those constructors are irrelevant. It's undefined to instantiate *any* std::lib template with an incomplete type unless *explicitly* permitted. It is explicitly permitted for vector and shared_ptr. It is not permitted for optional.
