An amazing read, and almost perfect! There's a bug in the Solve "function" due to which you'll get the right answer only for 1x1, 5x5 and 7x7 (I checked till 8

An amazing read, and almost perfect! There's a bug in the Solve "function" due to which you'll get the right answer only for 1x1, 5x5 and 7x7 (I checked till 8x8). The base case makes a wrong assumption that there will always be a candidate available for the last row. If there are no candidates available, it should return Nil, and backtrack. Basically, replace Concat<candidates, placedQueens> with candidates extends Cons<infer x, any> ? Cons<x, placedQueens> : Nil