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 E2E54383B420; Thu,  3 Jun 2021 13:22:22 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org E2E54383B420
From: "jlegg at feralinteractive dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100893] New: Template argument conversion fails for
 dependant constant function pointer template parameters
Date: Thu, 03 Jun 2021 13:22:22 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.1.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jlegg at feralinteractive dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-100893-4@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: Thu, 03 Jun 2021 13:22:23 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100893

            Bug ID: 100893
           Summary: Template argument conversion fails for dependant
                    constant function pointer template parameters
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlegg at feralinteractive dot com
  Target Milestone: ---

When compiling this C++ code:
void f();
struct S {using F =3D void (* const)();};
template <typename T, typename T::F f> void g() {}
void h() {g<S, &f>();}

GCC 11.1.1, and (much) earlier versions, reject it with:
<source>: In function 'void h()':
<source>:4:19: error: no matching function for call to 'g<S, f>()'
    4 | void h() {g<S, &f>();}
      |           ~~~~~~~~^~
<source>:3:45: note: candidate: 'template<class T, typename T::F f> void g(=
)'
    3 | template <typename T, typename T::F f> void g() {}
      |                                             ^
<source>:3:45: note:   template argument deduction/substitution failed:
<source>:4:16: error: could not convert template argument 'f' from 'void (*=
)()'
to 'void (* const)()'
    4 | void h() {g<S, &f>();}
      |                ^~

My language lawyering isn't good enough to explain why, but I think the
template argument conversion should work, and the compiler should behave as=
 it
does when the const is removed. Similar conversions seem to work if F is a
constant type but not a function pointer, or if the second template paramet=
er
on g is not dependant on the first. MSVC and Clang also accept the example
code.=