[Bug c++/94117] New: non-dependent expr treated as-if dependent

[Bug c++/94117] New: non-dependent expr treated as-if dependent

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

            Bug ID: 94117
           Summary: non-dependent expr treated as-if dependent
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 48006
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48006&action=edit
distilled from ranges

We accept the attached test case, which should be rejected.
It comes from these two fragments of the ranges header:

lines ~1595
          static constexpr decltype(auto)
          __iter_move(const _Iterator& __i = {})
            noexcept(noexcept(std::__invoke(*__i._M_parent->_M_fun,
                                            *__i._M_current)))
          {
            if constexpr (is_lvalue_reference_v<decltype(*__i)>)
              return std::move(*__i);
            else
              return *__i;
          }

lines ~1752
          friend constexpr decltype(auto)
          iter_move(const _Iterator& __i) noexcept(noexcept(__iter_move()))
          { return __iter_move(__i); }

If we change __iter_move's argument type to int, we reject complaining that the
call in iter_move's noexcept spec requires the default argument before it's
been parsed.  Clang rejects either way.

the '__iter_move ()' call is not dependent because it has no arguments. 
Addition of the default argument does not make it dependent.

filing a separate issue to fix libstdc++

[Bug c++/94117] non-dependent expr treated as-if dependent

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Related or dup of PR93901 ?  Guess I'll bisect to see if it is the same commit.

[Bug c++/94117] non-dependent expr treated as-if dependent

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Maybe not, as it doesn't seem to be a regression, it has never been rejected
(at least not in anything since r200000 I've tried).
clang++ rejects with
exception specification is not available until end of class definition

[Bug c++/94117] non-dependent expr treated as-if dependent

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Nathan Sidwell <nathan at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-03-10

--- Comment #3 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
This is the error that should be emitted (modulo the argument type being
_Iterator)

devvm2772:6>./cc1plus -fmodule-header -quiet b_a.H
In file included from b_a.H:1:
b.h:10:69: error: call to 'static void _Iterator<_Const>::mover(const int&)'
uses the default argument for parameter 1, which is not yet defined
   10 |   friend void move (const _Iterator &arg) noexcept (noexcept (mover
()))

[Bug c++/94117] non-dependent expr treated as-if dependent

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Nathan Sidwell from comment #0)
> filing a separate issue to fix libstdc++

Fixed in r10-7103-gc222eabcf8be0e3f644e4bd4c3316b40dba4b514

    libstdc++: Fix invalid noexcept-specifier (PR 94117)

    G++ fails to diagnose this non-dependent expression, but Clang doesn't
    like it.

            PR c++/94117
            * include/std/ranges
(ranges::transform_view::_Iterator::iter_move):
            Change expression in noexcept-specifier to match function body.

[Bug c++/94117] non-dependent expr treated as-if dependent

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh we have more occurrences of it in libstdc++, e.g. in split_view

          friend constexpr decltype(auto)
          iter_move(const _InnerIter& __i) noexcept(noexcept(__iter_move()))
          { return __iter_move(__i); }

          friend constexpr void
          iter_swap(const _InnerIter& __x, const _InnerIter& __y)
            noexcept(noexcept(__iter_swap()))
            requires indirectly_swappable<iterator_t<_Base>>
          { __iter_swap(__x, __y); }

[Bug c++/94117] non-dependent expr treated as-if dependent

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Nathan Sidwell <nathan at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |nathan at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #6 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
Ah, I understand what is happening.  

We don't defer parsing of the noexcept specified of friend functions.  That
leads us to construct a base-link expr whose type contains the deferred parse
exception specification of the *static* member function -- there being only
one, it doesn't get unknown type.

In processing the deferred exception specification of that fn, we don't fixup
all the type variants that might have been created.

There's a restriction on friend declarations that contain default args.  This
is the same problem that is trying to avoid.

[Bug c++/94117] deferred noexcept specifications and friend fns

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yes, it was a conscious decision not to defer parsing of noexcept-spec of
friend functions.  The standard currently doesn't say we shouldn't defer, but
no compiler implements it.  The problem was that friend declarations can be
redeclared and it would be tricky to see if their noexcept-specs match.

[Bug c++/94117] deferred noexcept specifications and friend fns

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> Oh we have more occurrences of it in libstdc++, e.g. in split_view

Fixed in r10-7108-gcf0c3a457319df1e8dc9321227162a7c57169a39

[Bug c++/94117] deferred noexcept specifications and friend fns

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Nathan Sidwell <nathan at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |SUSPENDED

--- Comment #9 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
Discussing on the core list  https://lists.isocpp.org/core/2020/03/8632.php
(password required)

I have a fix the the problem I encountered on the modules branch.  Not suitable
at this stage for trunk, and there's probably a better solution out there.

As Marek says 'nobody implements this'

[Bug c++/94117] deferred noexcept specifications and friend fns

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94117

Martin Liška <marxin at gcc dot gnu.org> changed:

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

--- Comment #10 from Martin Liška <marxin at gcc dot gnu.org> ---
commit r10-7103-gc222eabcf8be0e3f644e4bd4c3316b40dba4b514
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 10 10:50:40 2020 +0000

    libstdc++: Fix invalid noexcept-specifier (PR 94117)

    G++ fails to diagnose this non-dependent expression, but Clang doesn't
    like it.

            PR c++/94117
            * include/std/ranges
(ranges::transform_view::_Iterator::iter_move):
            Change expression in noexcept-specifier to match function body.