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From: "richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99176] New: GCC rejects const_cast of null pointer in
 constant expressions
Date: Fri, 19 Feb 2021 23:11:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99176

            Bug ID: 99176
           Summary: GCC rejects const_cast of null pointer in constant
                    expressions
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

GCC rejects:

constexpr const int *p =3D nullptr;
constexpr int *q =3D const_cast<int*>(p);

saying:

<source>:2:20: error: conversion of 'const int*' null pointer to 'int*' is =
not
a constant expression
    2 | constexpr int *q =3D const_cast<int*>(p);
      |                    ^~~~~~~~~~~~~~~~~~~

I don't think any such rule exists, and other compilers accept. This only
appears to affect const_casts of null pointers; non-null pointer const casts
seem to work OK. Perhaps GCC thinks that this is a reinterpret_cast / cast =
from
void* or something like that?

It looks like this regressed between GCC 6 and GCC 7.=