[v3 PATCH] PR libstdc++/70437

[Jonathan Wakely]

On 04/04/16 21:45 +0300, Ville Voutilainen wrote:
>And yes, -ENOPATCH.
>
>On 4 April 2016 at 21:42, Ville Voutilainen <ville.voutilainen@gmail.com> wrote:
>> Tested on Linux-PPC64.
>>
>> 2016-04-04  Ville Voutilainen  <ville.voutilainen@gmail.com>
>>
>>      PR libstdc++/70437
>>      * include/bits/stl_pair.h (_ConstructiblePair,
>>     _ImplicitlyConvertiblePair, _MoveConstructiblePair,
>>     _ImplicitlyMoveConvertiblePair): Add shortcut conditions
>>     for same-type cases.
>>     * testsuite/20_util/pair/70437.cc: New.

Thanks for the fix.

>diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
>index 7057030..206553a 100644
>--- a/libstdc++-v3/include/bits/stl_pair.h
>+++ b/libstdc++-v3/include/bits/stl_pair.h
>@@ -90,29 +90,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   template <typename _T1, typename _T2, typename _U1, typename _U2>
>   constexpr bool _ConstructiblePair()
>   {
>-    return __and_<is_constructible<_T1, const _U1&>,
>-		  is_constructible<_T2, const _U2&>>::value;
>+    return __and_<__or_<is_same<typename decay<_T1>::type,
>+				typename decay<_U1>::type>,
>+			is_constructible<_T1, const _U1&>>,
>+		  __or_<is_same<typename decay<_T2>::type,
>+				typename decay<_U2>::type>,
>+			is_constructible<_T2, const _U2&>>>::value;
>   }

I wonder if we want an __is_samey trait that checks if two decayed
types are the same.

More seriously, a comment might be useful to explain that although
these "concepts" return true for samey types, that is just to prevent 
is_constructible from getting into a mess with incomplete types, and
actually for samey types one of the special member functions might end
up being chosen by overload resolution instead.

Did I get that right? If so, I definitely think it's worth a comment,
as I for one won't remember the details in a few months!