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From: "peifeng2005 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97432] Casting away constness in the drafts [conv.qual] a cv-decomposition exists in the following examples but fails in gcc
Date: Thu, 15 Oct 2020 00:12:17 +0000 [thread overview]
Message-ID: <bug-97432-4-k0iKtQ77pU@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-97432-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97432
--- Comment #1 from Ray Zhang <peifeng2005 at gmail dot com> ---
Providing more context with the first example (also applies for the rest):
1) // pointer to array - works in clang (trunk),
// works in gcc (trunk)
reinterpret_cast<int* const T::* const (*) [2]>(x);
T1: const pointer to const pointer to const pm(T) to const pointer to const U1
T2: pointer to const array of 2 const pm(T) to const pointer to U2
In this case, there exists multiple cv-decompositions, as [7.3.6].1 example 1
states:
> [Example 1: The type denoted by the type-id const int ** has three cv-decompositions, taking U as “int”, as “pointer to const int”, and as “pointer to pointer to const int”.
The cv-decompositions we have here for T1 is:
- const pointer to const U1
- const pointer to const pointer to const U1
- const pointer to const pointer to const pm(T) to const U1
- const pointer to const pointer to const pm(T) to const pointer to const U1
If we fulfill one such cv-decompositions, we can consider that reinterpret_cast
will not cast away constness, though U1 and U2 may be nested pointer/array
types of interest.
The unique cv-decomposition that a compiler can decide to take of T1 is:
- const pointer to const U1, where U1 is "pointer to const pm(T) to const
pointer to const int"
The cv-decomposition on T2 can be done similarly:
- const pointer to const U2, where U2 is "array of 2 const pm(T) to const
pointer to int"
Thus, we can perform a reinterpret cast from U1 to U2 without casting away
constness.
next prev parent reply other threads:[~2020-10-15 0:12 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-15 0:10 [Bug c++/97432] New: " peifeng2005 at gmail dot com
2020-10-15 0:12 ` peifeng2005 at gmail dot com [this message]
2020-10-15 8:06 ` [Bug c++/97432] " peifeng2005 at gmail dot com
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