>From what Gregory and Anton have said, it sounds like the SockAddr6,

HostAddress6 constructors should always be available, but that using
them with the socket functions should fail at runtime on platforms with
no IPv6 support, or when IPv6 support is turned off for some reason.

That would probably make it rather easier to have portable programs that
can optionally use IPv6. Having to use TH to test at compile time if a
constructor is or is not exported from another package is more than a
little unpleasant.