[Patch] [add changelog] reduce template instantiation depth in <variant>

[Jonathan Wakely]

I've rebased Barrett's patch against the latest variant code
(attached).

Tim, do we want to make this change? A variant of 400 alternatives
seems pretty extreme, but if we can support it without much trouble
then maybe it's good to do so.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.txt
Type: text/x-patch
Size: 3056 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20161206/3f73a1de/attachment.bin>