Proposed resolution:

This wording is relative to N4835.

1. Modify [util.smartptr.shared.const] as indicated:

   template<class Y, class D> shared_ptr(Y* p, D d);
   template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   template<class D> shared_ptr(nullptr_t p, D d);
   template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
   

   -?- Constraints: is_move_constructible_v<D> is true, and d(p) is a well-formed expression. For the first two overloads:

   • If T is an array type, then either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.

   • If T is not an array type, then Y* is convertible to T*.

   -9- RequiresExpects: Construction of d and a deleter of type […]

   […]

   -13- Remarks: When T is an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and Y* is convertible to T*.

[ 2019-11-17 Issue Prioritization ]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Issue 2875 added [util.smartptr.shared.const] paragraph 13:

Remarks: When T is an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless is_move_constructible_v<D> is true, the expression d(p) is well-formed, and Y* is convertible to T*.

template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

MSVC ignores the constraints on Y for the overloads with no such template parameter, whereas libstdc++ and libc++ seem to ignore all of the constraints for those overloads (See Compiler Explorer). We should fix the broken wording, ideally by requiring the MSVC interpretation - the nullptr_t constructors participate in overload resolution only when is_movable_v<D> is true and d(p) is well-formed - so concepts and traits that check constructibility are correct.