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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) 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`.