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From: "mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/65642] [C++11] GCC rejects valid constant expression
Date: Tue, 07 Apr 2015 17:19:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65642

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Assignee|mpolacek at gcc dot gnu.org        |unassigned at gcc dot gnu.org
   Target Milestone|5.0                         |6.0

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I think that fixing the third case is going to be harder, and doesn't look like
something that should be fixed in cp/constexpr.c.

While fold_comparison is able to fold the NE_EXPR here:
constexpr char s1[] = "s1";
constexpr char s2[] = "s2";
constexpr auto eq = (s2 + 2) != (s2 + 1);

it isn't able to fold it here:
constexpr char s1[] = "s1";
constexpr char s2[] = "s2";
constexpr auto eq = (s1 + 2) != (s2 + 1);

So not sure what to do here, but in any case, I think it is a GCC 6 material.