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 9F739397B80E; Thu, 15 Jul 2021 10:56:43 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 9F739397B80E
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96286] Unhelpful errors after a failed static_assert
Date: Thu, 15 Jul 2021 10:56:43 +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: 10.1.1
X-Bugzilla-Keywords: diagnostic
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: NEW
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: 
Message-ID: <bug-96286-4-4TpxJmecRU@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-96286-4@http.gcc.gnu.org/bugzilla/>
References: <bug-96286-4@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, 15 Jul 2021 10:56:43 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96286

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It also doesn't help that the compiler thwarts my attempt to trigger errors
earlier like so:

  // Return index of _Tp in _Types.
  // Ill-formed if _Tp doesn't occur exactly once in _Types.
  template<typename _Tp, typename... _Types>
    constexpr size_t
    __find_uniq_type_in_pack()
    {
      constexpr ptrdiff_t __idx =3D std::__find_type_in_pack<_Tp, _Types...=
>();
      static_assert(__idx >=3D 0,
          "the type T in std::get<T> must occur exactly once in the tuple");
      return size_t{__idx};
    }

The return statement *should* be an ill-formed narrowing conversion, but G++
(and Clang) think that because I'm in a system header we probably wanted to
ignore that, so we return a huge value and get 200 lines of errors again.

Maybe this will work?
    {
      constexpr ptrdiff_t __idx =3D std::__find_type_in_pack<_Tp, _Types...=
>();
      static_assert(__idx >=3D 0,
          "the type T in std::get<T> must occur exactly once in the tuple");
      if (__idx >=3D 0)
        return __idx;
      __builtin_unreachable();
    }

No, now I get 200 lines telling me that the I in std::get<I> is not a const=
ant
expression because of __builtin_unreachable.

OK, let's try       return enable_if_t<__idx >=3D 0, size_t>{__idx};

This prints "error: no type named 'type' in 'struct std::enable_if'" as
expected, but then 200 lines telling me std::get<I> is not a constant
expression because it flows off the end of the function.

Why is the compiler even doing overload resolution for std::get<I> if I is =
an
invalid constant expression?! It's not going to match any overload!

It shouldn't be this hard. Just stop compiling already.=