This is a bit challenging to phrase, so I'll use an example.
A human wizard with the Tavern Brawler feat gets fed up with a unruly Gnome and snaps off a heavy gate post to stab him with. The DM rules that the post is similar enough to a pike to use its damage dice. Is the Wizard proficient in this improvised Pike?
A related question queries whether or not an improvised weapon deemed similar enough to an actual weapon is still improvised - the consensus was that it was.
If that's the case, the Tavern Brawler feat states:
You are proficient with improvised weapons.
If this too is the case, does this create a loophole of sorts where a character can wield an improvised variant of a weapon they otherwise couldn't use proficiently?