RFC defining std::function::assign

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

void function::assign(F&&, const Alloc& a);

Currently this function is not defined because it would need to use
the function(allocator_arg_t, const Alloc&, F) constructor, which
doesn't exist either, because we don't have allocator_arg_t yet.

The patch below would define function::assign, but ignore the
allocator argument. That allows the attached test to compile, but it's
XFAILed because the allocator isn't used.

I think it's better to have the assign function, even if it doesn't
use the allocator, but would like to hear what others think.

Attachment: funcalloc.txt
Description: Text document