psql was already accepting conninfo strings as the first parameter in \connect, but the way it worked wasn't sane; some of the other parameters would get the previous connection's values, causing it to connect to a completely unexpected server or, more likely, not finding any server at all because of completely wrong combinations of parameters.
I haven't looked at that in details, but it looks that there are linking problems with -lpgcommon, as it is the first time that libpq has a dependency with it.
I am seeing as well that this is at least missing at the top of connstring.c: #ifndef FRONTEND #error "This file is not expected to be compiled for backend code" #endif
And that src/tools/msvc has not been updated to have connstring.c show up in the list of frontend-only objects for libpgcommon.
