vsapsai added inline comments.
================
Comment at: libcxx/include/memory:1479
+struct __has_construct_missing
+ : false_type
+{
----------------
erik.pilkington wrote:
> vsapsai wrote:
> > erik.pilkington wrote:
> > > Shouldn't this be true_type?
> > I see this is confusing and I'm still struggling how to express it. The
> > issue is that in C++03 `__has_construct` should be something like unknown,
> > so that neither `__has_construct` nor `! __has_construct` evaluate to true
> > because we don't really know if allocator has construct. This case is
> > covered by the added test, in C++03 the memcpy specialization was enabled
> > when
> >
> > ```
> > is_same<allocator_type, allocator<_Tp> >
> > || !false_type
> > ```
> >
> > So `is_same` check had no effect and we were using memcpy to convert
> > between int and float.
> >
> > I was considering using something like
> >
> > ```lang=c++
> > typename enable_if
> > <
> > (is_same
> > <
> > typename _VSTD::remove_const<typename
> > allocator_type::value_type>::type,
> > typename _VSTD::remove_const<_SourceTp>::type
> > >::value
> > #ifndef _LIBCPP_CXX03_LANG
> > || !__has_construct<allocator_type, _DestTp*,
> > _SourceTp>::value
> > #endif
> > ) &&
> > is_trivially_move_constructible<_DestTp>::value,
> > void
> > >::type
> > ```
> >
> > But that doesn't look readable to me, so I've introduced ad-hoc ternary
> > logic with `__has_construct_missing`.
> Oh I see, yikes! That's a pretty bad bug. I agree that this fix is best then,
> but can you add a comment explaining this to `__has_construct_missing` for
> future casual readers? Also, I think we should move the
> __has_construct_missing bugfix into a different (prerequisite) patch. Seems
> unrelated to the `const` related optimization below.
The bug as I described isn't really present now because function signature
__construct_range_forward(allocator_type&, _Tp* __begin1, _Tp* __end1,
_Tp*& __begin2)
works as implicit `is_same` for `__begin1` and `__begin2` types. I think it is
worth fixing separately and there is a bug with C++03 and custom allocators.
https://reviews.llvm.org/D48342
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