[async.reqmts.proto.allocator] "No constructor ... shall exit via an exception" over-specification

[chriskohlhoff]

This is probably a copy/paste error introduced when this text was copied from the standard (I believe from [allocator.requirements]).

The intent is only that copy and move constructors shall not throw, and I think that is already covered by "copy operation, move operation". Other constructors not covered by the ProtoAllocator requirements should be allowed to throw.

Suggested resolution: Delete "constructor," so that the sentence reads "No comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception."