[Bug c++/100384] New: Compiling in c++17 mode breaks compilation of functions named visit()

[Bug c++/100384] New: Compiling in c++17 mode breaks compilation of functions named visit()

[aacid at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

            Bug ID: 100384
           Summary: Compiling in c++17 mode breaks compilation of
                    functions named visit()
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aacid at kde dot org
  Target Milestone: ---

Created attachment 50733
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50733&action=edit
main.cpp

The attached code fails to compile with 

g++ --std=c++17 main.cpp 

I've tried with gcc 11.1 too and it fails too.

To me it seems a bug in the implementation in which std::visit is leaking into
the non std:: namespace, but maybe it's a bug in C++ specification?

[Bug c++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[aacid at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #1 from Albert Astals Cid <aacid at kde dot org> ---
Created attachment 50734
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50734&action=edit
error log

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[aacid at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #2 from Albert Astals Cid <aacid at kde dot org> ---
People that know more C++ than me, told me

std::visit appears unconstrained to Variants...&& being actual std::variants
(in case you inherited one, I guess), so std::visit, as an unconstrained
perfect forwarder will win every overload set ranking. And std::visit is
brought in by ADL via std::vector

it's a misfeature, this has been adopted in C++23
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2162r2.html 

So I guess the proper resolution for this issue would be "not a bug", but will
leave you to decide on it :)

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Not a bug.

PR 90943 already exists for the P2162R3 support.

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Hmm, isn't problem here ultimately that std::visit is specified to have a
concrete SFINAE-friendly return type, but our implementation uses a deduced
return type?

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Possibly, but it needs to be constrained for p2162 anyway, which I'm doing via
the return type.

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |---
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-03
             Status|RESOLVED                    |NEW

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, we can fix this easily without the rest of P2162 (and can backport it):

--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1248,7 +1248,8 @@ namespace __variant
 #endif

   template<typename _Visitor, typename... _Variants>
-    constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);
+    constexpr invoke_result_t<_Visitor, variant_alternative_t<0,
_Variants>...>
+    visit(_Visitor&&, _Variants&&...);

   template<typename... _Types>
     inline enable_if_t<(is_move_constructible_v<_Types> && ...)
@@ -1736,7 +1737,7 @@ namespace __variant


   template<typename _Visitor, typename... _Variants>
-    constexpr decltype(auto)
+    constexpr invoke_result_t<_Visitor, variant_alternative_t<0,
_Variants>...>
     visit(_Visitor&& __visitor, _Variants&&... __variants)
     {
       if ((__variants.valueless_by_exception() || ...))

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:af5b2b911dd80ae9cc87404b7e7ab807cf6655d4

commit r12-436-gaf5b2b911dd80ae9cc87404b7e7ab807cf6655d4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 12:16:46 2021 +0100

    libstdc++: Do not use deduced return type for std::visit [PR 100384]

    This avoids errors outside the immediate context when std::visit is an
    overload candidate because of ADL, but not actually viable.

    The solution is to give std::visit a non-deduced return type. New
    helpers are introduced for that, and existing ones refactored slightly.

    libstdc++-v3/ChangeLog:

            PR libstdc++/100384
            * include/std/variant (__get_t): New alias template yielding the
            return type of std::get<N> on a variant.
            (__visit_result_t): New alias template yielding the result of
            std::visit.
            (__same_types): Move into namespace __detail::__variant.
            (__check_visitor_results): Likewise. Use __invoke_result_t and
            __get_t.
            (__check_visitor_result): Remove.
            (visit): Use __visit_result_t for return type.
            * testsuite/20_util/variant/100384.cc: New test.

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:e99763ee6da8e378073b847243d9ac2538903534

commit r11-8363-ge99763ee6da8e378073b847243d9ac2538903534
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 12:16:46 2021 +0100

    libstdc++: Do not use deduced return type for std::visit [PR 100384]

    This avoids errors outside the immediate context when std::visit is an
    overload candidate because of ADL, but not actually viable.

    The solution is to give std::visit a non-deduced return type. New
    helpers are introduced for that, and existing ones refactored slightly.

    libstdc++-v3/ChangeLog:

            PR libstdc++/100384
            * include/std/variant (__get_t): New alias template yielding the
            return type of std::get<N> on a variant.
            (__visit_result_t): New alias template yielding the result of
            std::visit.
            (__same_types): Move into namespace __detail::__variant.
            (__check_visitor_results): Likewise. Use __invoke_result_t and
            __get_t.
            (__check_visitor_result): Remove.
            (visit): Use __visit_result_t for return type.
            * testsuite/20_util/variant/100384.cc: New test.

    (cherry picked from commit af5b2b911dd80ae9cc87404b7e7ab807cf6655d4)

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:8ad4d9b46944db1be6f1b216b5b8e74bd9f66937

commit r10-9937-g8ad4d9b46944db1be6f1b216b5b8e74bd9f66937
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 12:16:46 2021 +0100

    libstdc++: Do not use deduced return type for std::visit [PR 100384]

    This avoids errors outside the immediate context when std::visit is an
    overload candidate because of ADL, but not actually viable.

    The solution is to give std::visit a non-deduced return type. New
    helpers are introduced for that, and existing ones refactored slightly.

    libstdc++-v3/ChangeLog:

            PR libstdc++/100384
            * include/std/variant (__get_t): New alias template yielding the
            return type of std::get<N> on a variant.
            (__visit_result_t): New alias template yielding the result of
            std::visit.
            (__same_types): Move into namespace __detail::__variant.
            (__check_visitor_results): Likewise. Use __invoke_result_t and
            __get_t.
            (__check_visitor_result): Remove.
            (visit): Use __visit_result_t for return type.
            * testsuite/20_util/variant/100384.cc: New test.

    (cherry picked from commit af5b2b911dd80ae9cc87404b7e7ab807cf6655d4)

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:91d29ed563bd7e787921f997ea2f80cd87ee59b2

commit r9-9592-g91d29ed563bd7e787921f997ea2f80cd87ee59b2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 12:16:46 2021 +0100

    libstdc++: Do not use deduced return type for std::visit [PR 100384]

    This avoids errors outside the immediate context when std::visit is an
    overload candidate because of ADL, but not actually viable.

    The solution is to give std::visit a non-deduced return type. New
    helpers are introduced for that.

    libstdc++-v3/ChangeLog:

            PR libstdc++/100384
            * include/std/variant (__get_t): New alias template yielding the
            return type of std::get<N> on a variant.
            (__visit_result_t): New alias template yielding the result of
            std::visit.
            (__do_visit): Use __get_t.
            (visit): Use __visit_result_t for return type.
            * testsuite/20_util/variant/100384.cc: New test.

    (cherry picked from commit af5b2b911dd80ae9cc87404b7e7ab807cf6655d4)

[Bug libstdc++/100384] Compiling in c++17 mode breaks compilation of functions named visit()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100384

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |9.5

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 9.5, 10.4 and 11.2