Proposed resolution:

This resolution is relative to N4659.

1. Edit [util.smartptr.weak], class template weak_ptr synopsis, as indicated:

   template<class T> class weak_ptr {
   public:
     using element_type = remove_extent_t<T>;
     […]
   };
   

[ 2017-07-17 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

C++17's shared_ptr<T>::element_type is remove_extent_t<T>, but weak_ptr<T>::element_type is T. They should be the same, but this was lost over time.

First, N4562 "Working Draft, C++ Extensions for Library Fundamentals, Version 2" 8.2.2 [memory.smartptr.weak] specified:

namespace std {
namespace experimental {
inline namespace fundamentals_v2 {

  template<class T> class weak_ptr {
  public:
    typedef typename remove_extent_t<T> element_type;

(The typename here was spurious.)

Then, P0220R1 "Adopt Library Fundamentals V1 TS Components for C++17 (R1)" listed:

• 8.2.2 Class template weak_ptr

  • 8.2.2.1 weak_ptr constructors

This obscured the fact that the Library Fundamentals TS had altered weak_ptr::element_type.

Finally, P0414R2 "Merging shared_ptr changes from Library Fundamentals to C++17" missed the change to weak_ptr::element_type, so it wasn't applied to C++17.

Peter Dimov has confirmed that this was unintentionally lost, and that "boost::weak_ptr defines element_type in the same way as shared_ptr".