Seems to be a DMD 2.067.0 regression (still doesn't work in 2.067.1, but
works for 2.066.1). Unfortunately it's always a but nasty to reduce such
cross-library regressions... you'd have to copy the relevant code form
phobos into a separate DUB package and change the code in vibe.http.client to use that version. Then "dub dustmite" should do
the trick.
I don't have time to look into this now, but in general I'd eventually
like to replace that regex code by something more concise anyway (also
to get compile times down). That IPv6 regex is really ridiculously large.