From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 43CCA3893679; Fri,  5 Mar 2021 15:30:56 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 43CCA3893679
From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99287] std::source_location::function_name breaks constexpr
Date: Fri, 05 Mar 2021 15:30:56 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ppalka at gcc dot gnu.org
X-Bugzilla-Status: ASSIGNED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: ppalka at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: assigned_to bug_status
Message-ID: <bug-99287-4-tujucpvssq@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-99287-4@http.gcc.gnu.org/bugzilla/>
References: <bug-99287-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 05 Mar 2021 15:30:56 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99287

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot g=
nu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
IIUC, those two types are actually the same, it's just that one of them was
obtained through the char_type alias, and it seems debug_tree prefers to sh=
ow
the name of the alias when the type came from one.

It seems the issue ultimately is in cxx_eval_increment_expression: during
evaluation of ++__first with __first =3D &"mystr"[n] + m and with lval=3Dfa=
lse,
since cxx_fold_pointer_plus_expression and not fold_build2 is responsible f=
or
simplifying this POINTER_PLUS_EXPR to &"mystr"[n+m], returning 'mod' actual=
ly
returns the unreduced &"mystr"[n] + m rather than the reduced &"mystr"[n+m]
that we obtain as part of constexpr evaluation of the temporary MODIFY_EXPR=
.=20
This unreduced return value of cxx_eval_increment interfers with later
constexpr evaluation, e.g. folding of the POINTER_DIFF_EXPR.

I'm testing the following which updates 'mod' with the result of evaluation=
 of
the MODIFY_EXPR:

--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5582,20 +5582,14 @@ cxx_eval_increment_expression (const constexpr_ctx
*ctx, tree t,
   /* Storing the modified value.  */
   tree store =3D build2_loc (cp_expr_loc_or_loc (t, input_location),
                           MODIFY_EXPR, type, op, mod);
-  cxx_eval_constant_expression (ctx, store,
-                               true, non_constant_p, overflow_p);
+  mod =3D cxx_eval_constant_expression (ctx, store, false,
+                                     non_constant_p, overflow_p);
   ggc_free (store);

   /* And the value of the expression.  */=