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 8079D383F874; Mon, 11 May 2020 17:50:04 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 8079D383F874
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1589219404;
 bh=fgQcqY1u3ve32isN7qrnhzovnQtTY7G/08c9dreK2h4=;
 h=From:To:Subject:Date:From;
 b=HjcfHaWEG9VKGa8xL9vZQcW02hvhRGuUU+STaZjKT1o7L/FsvNMh56cHvhl09zM34
 U3JclIW1RZ2ie18L6/2UjLxUXZms79iUUnale7/3yUcG3raj0bx3ZlpG2c4+Ec9ZmK
 soNke4545nydl/2R+DY1c311Oubuy0FglSeOc5Wc=
From: "ojman101 at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95066] New: [C++ 20] Incorrect valid compilation with a
 conditional explicit
Date: Mon, 11 May 2020 17:50:04 +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: 9.3.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ojman101 at protonmail 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-95066-4@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: <http://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: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Mon, 11 May 2020 17:50:04 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95066

            Bug ID: 95066
           Summary: [C++ 20] Incorrect valid compilation with a
                    conditional explicit
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ojman101 at protonmail dot com
  Target Milestone: ---

The code below is invalid C++, the line "Foo<int> b =3D a;" should fail to
compile as implicitly casting is made illegal by the conditional explicit u=
sing
the "IsSafelyCastable" predicate.

----------------------------------------------------------------
#include <type_traits>

template <typename, typename>
class IsSafelyCastable : public std::false_type {};

template <>
class IsSafelyCastable<int, float> : public std::true_type {};

template <typename T>
struct Foo {
    template <typename U>
    explicit(!IsSafelyCastable<T, U>::value) operator Foo<U>();
};

template <typename T>
template <typename U>
Foo<T>::operator Foo<U>() {
  return {};
}

int main() {
    Foo<float> a;
    Foo<int> b =3D a;
}
----------------------------------------------------------------

Clang 10 correctly evaluates the explicit condition to be true and blocks t=
he
implicit cast. However, GCC 9.3.0 successfully compiles without any errors.=
 I
believe this to be a GCC bug as subtle changes can make GCC produce the cor=
rect
error. For example, moving the definition of the function to be inline with=
 the
declaration.=