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2859. Definition of reachable in [ptr.launder] misses pointer arithmetic from pointer-interconvertible object

Section: 17.6.5 [ptr.launder] Status: C++20 Submitter: Hubert Tong Opened: 2017-01-31 Last modified: 2021-02-25

Priority: 2

View all other issues in [ptr.launder].

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Discussion:

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.

[2017-03-04, Kona]

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

[2017-08-14, CWG telecon note]

CWG is fine with the proposed resolution.

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

Proposed resolution:

This wording is relative to N4618.

  1. Modify 17.6.5 [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.