[v3 PATCH] PR libstdc++/79141

[Jonathan Wakely]

On 02/04/17 02:45 +0300, Ville Voutilainen wrote:
>    PR libstdc++/79141
>    * include/bits/stl_pair.h (__wrap_nonesuch): New.
>    (operator=(typename conditional<
>    __and_<is_copy_assignable<_T1>,
>    is_copy_assignable<_T2>>::value,
>    const pair&, const __wrap_nonesuch&>::type)): Change __nonesuch
>    to __wrap_nonesuch.
>    (operator=(typename conditional<
>    __not_<__and_<is_copy_assignable<_T1>,
>    is_copy_assignable<_T2>>>::value,
>    const pair&, const __nonesuch&>::type)): Likewise.
>    (operator=(typename conditional<
>    __and_<is_move_assignable<_T1>,
>    is_move_assignable<_T2>>::value,
>    pair&&, __wrap_nonesuch&&>::type)): Likewise.
>    * testsuite/20_util/pair/79141.cc: New.

>diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
>index 7c7cee2..b746fb4 100644
>--- a/libstdc++-v3/include/bits/stl_pair.h
>+++ b/libstdc++-v3/include/bits/stl_pair.h
>@@ -179,6 +179,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       }
>   };
>
>+  struct __wrap_nonesuch : std::__nonesuch {
>+    explicit __wrap_nonesuch(const __nonesuch&) = delete;
>+  };

Could you please add a comment explaining that this is needed to
ensure that functions with parameters of this type are not viable when
an argument of {} is used. Because even with such a comment I'll
probably not understand this by next week.

And I think __explicit_nonesuch or __no_list_init would be a clearer
name. We're not really "wrapping" this, unless I misunderstand.

OK for trunk, I suppose.


This kind of thing is why I want to burn std::pair and std::tuple to
the ground.