Proposed resolution:

This wording is relative to N4618.

1. Modify [ptr.launder] as indicated:

   template <class T> constexpr T* launder(T* p) noexcept;
   

   […]

   -3- Remarks: An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression. A byte of storage b is reachable through a pointer value that points to an object Y if there is an object Z, pointer-interconvertible with Y, such that b it is within the storage occupied by ZY, an object that is pointer-interconvertible with Y, or the immediately-enclosing array object if ZY is an array element. The program is ill-formed if T is a function type or (possibly cv-qualified) void.

[ 2020-02 Status to Immediate on Thursday night in Prague. ]

[ 2017-08-14, CWG telecon note ]

CWG is fine with the proposed resolution.

[ 2017-03-04, Kona ]

Set priority to 2. Assign this (and 2860) to Core.

Given:

struct A { int x, y; };
A a[100];

The bytes which compose a[3] can be reached from &a[2].x: reinterpret_cast<A *>(&a[2].x) + 1 points to a[3], however, the definition of "reachable" in [ptr.launder] does not encompass this case.