在 2022/12/2 08:17, leon zadorin via Gcc-help 写道:
> Thanks Jonathan for clarifying, as always - very precise and helpful. I'll
> close the issue on the clang side also.

No, you do not _instantiate_ `J<X>`. The behavior would have been undefined if you did. 
`::std::shared_ptr<J<X>> p;` is fine on itself, because `J<X>` is not instantiated. On the other 
hand, `::std::shared_ptr<J<X>> p(static_cast<J<X>*>(nullptr));` is undefined, as it instantiates a 
delete expression.


In addition to that, [temp.point]/1 specifies that the point of instantiation of members of a class 
template _follows the end of_ namespace scope of the most enclosing specialization. This allows use 
before definition, which is not permitted for non-template classes:

    std::shared_ptr<struct foo> ptr(
          (struct foo*) nullptr);  // instantiates `delete (struct foo*) ___`

    struct foo { };  // the line above would have undefined behavior without this


So, either way, your code is not undefined.


-- 
Best regards,
LIU Hao