[Bug libstdc++/97449] New: libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[Bug libstdc++/97449] New: libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[foom at fuhm dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

            Bug ID: 97449
           Summary: libstdc++ <variant> cannot be compiled with clang
                    after 3427e31331677ca826c5588c87924214f7e5c54b
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
  Target Milestone: ---

After 3427e31331677ca826c5588c87924214f7e5c54b, "clang -std=c++17" (using
libstdc++ headers), on a file that has simply "#include <variant>" fails with
this error:

In file included from <source>:1:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/variant:1032:10:
error: no matching constructor for initialization of 'std::__nonesuch'
                return __nonesuch{};
                       ^         ~~

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/type_traits:2953:5:
note: candidate constructor not viable: requires 1 argument, but 0 were
provided
    __nonesuch(__nonesuch const&) = delete;
    ^

(https://godbolt.org/z/951jYr)

I believe Clang is correct to reject this code, due to
http://eel.is/c++draft/temp.res#general-8.1
"""
The program is ill-formed, no diagnostic required, if:
- no valid specialization can be generated for a template or a substatement of
a constexpr if statement within a template and the template is not instantiated
"""

The invalid return statement probably needs to be made dependent on
__visit_ret_type_mismatch just like the static_assert.

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.0
      Known to fail|                            |11.0
            Summary|libstdc++ <variant> cannot  |[11 Regression] libstdc++
                   |be compiled with clang      |<variant> cannot be
                   |after                       |compiled with clang after
                   |3427e31331677ca826c5588c879 |3427e31331677ca826c5588c879
                   |24214f7e5c54b               |24214f7e5c54b
                 CC|                            |ville at gcc dot gnu.org
           Keywords|                            |rejects-valid
      Known to work|                            |10.2.1

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-10-15
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |ville.voutilainen at gmail dot com
           Assignee|unassigned at gcc dot gnu.org      |ville.voutilainen at gmail dot com

--- Comment #1 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Mine. I have a patch for it already, doing testing and submission chores...

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

--- Comment #2 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Patch available:
https://gcc.gnu.org/pipermail/gcc-patches/2020-October/556323.html

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Ville Voutilainen <ville@gcc.gnu.org>:

https://gcc.gnu.org/g:1f65bf2aa65609c0cd88af1b83191d37d6729f46

commit r11-4024-g1f65bf2aa65609c0cd88af1b83191d37d6729f46
Author: Ville Voutilainen <ville.voutilainen@gmail.com>
Date:   Sat Oct 17 22:08:50 2020 +0300

    libstdc++: Fix visitor return type diagnostics [PR97449]

    libstdc++-v3/ChangeLog:

            PR libstdc++/97449
            * include/std/variant
            (__gen_vtable_impl<>::_S_apply_single_alt):
            Diagnose visitor return type mismatches here..
            (__gen_vtable_impl</*base case*/>::_S_apply):
            ..not here.

[Bug libstdc++/97449] [11 Regression] libstdc++ <variant> cannot be compiled with clang after 3427e31331677ca826c5588c87924214f7e5c54b

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97449

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Fixed.