union validity check happens too early

[hishamhm]

local interface A
   where true
end

local record D
   v: A | C
end

local interface C
   where true
end

local d: D

local v = d.v
if v is A then
end

The union in D.v is declared invalid because at the time of check, C's where clause was not declared yet. Declaring C above D makes the error go away.