[PATCH][_Hashtable] Fix insertion of range of type convertible to value_type PR 56112

[Jonathan Wakely]

On Tue, 24 May 2022 at 11:18, Jonathan Wakely wrote:
>
> On Thu, 5 May 2022 at 18:38, François Dumont via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Hi
> >
> > Renewing my patch to fix PR 56112 but for the insert methods, I totally
> > change it, now works also with move-only key types.
> >
> > I let you Jonathan find a better name than _ValueTypeEnforcer as usual :-)
> >
> > libstdc++: [_Hashtable] Insert range of types convertible to value_type
> > PR 56112
> >
> > Fix insertion of range of types convertible to value_type. Fix also when
> > this value_type
> > has a move-only key_type which also allow converted values to be moved.
> >
> > libstdc++-v3/ChangeLog:
> >
> >          PR libstdc++/56112
> >          * include/bits/hashtable_policy.h (_ValueTypeEnforcer): New.
> >          * include/bits/hashtable.h
> > (_Hashtable<>::_M_insert_unique_aux): New.
> >          (_Hashtable<>::_M_insert(_Arg&&, const _NodeGenerator&,
> > true_type)): Use latters.
> >          (_Hashtable<>::_M_insert(_Arg&&, const _NodeGenerator&,
> > false_type)): Likewise.
> >          (_Hashtable(_InputIterator, _InputIterator, size_type, const
> > _Hash&, const _Equal&,
> >          const allocator_type&, true_type)): Use this.insert range.
> >          (_Hashtable(_InputIterator, _InputIterator, size_type, const
> > _Hash&, const _Equal&,
> >          const allocator_type&, false_type)): Use _M_insert.
> >          * testsuite/23_containers/unordered_map/cons/56112.cc: Check
> > how many times conversion
> >          is done.
> >          (test02): New test case.
> >          * testsuite/23_containers/unordered_set/cons/56112.cc: New test.
> >
> > Tested under Linux x86_64.
> >
> > Ok to commit ?
>
> No, sorry.
>
> The new test02 function in 23_containers/unordered_map/cons/56112.cc
> doesn't compile with libc++ or MSVC either, are you sure that test is
> valid? I don't think it is, because S2 is not convertible to
> pair<const MoveOnlyKey, int>. None of the pair constructors are
> viable, because the move constructor would require two user-defined
> conversions (from S2 to pair<MoveOnlyKey, int> and then from
> pair<MoveOnlyKey, int> to pair<const MoveOnlyKey, int>). A conversion
> sequence cannot have more than one user-defined conversion using a
> constructor or converion operator. So if your patch makes that
> compile, it's a bug in the new code. I haven't analyzed that code to
> see where the problem is, I'm just looking at the test results and the
> changes in behaviour.

I meant to include this link showing that libc++ and MSVC reject
test02() as well:

https://godbolt.org/z/j7E9f6bd4


>
> The new 23_containers/unordered_set/cons/56112.cc test fails for GCC
> 11 but passes for GCC 12, even without your patch. Is it actually
> testing some other change, not this patch, and not the 2013 fix for PR
> 56112?