I noticed that the following FPC code (claimed to be BP compatible) is not accepted by GPC:
function b:string; begin b[1] := ' ' end;
Similarely FPC accepts:
type a = record i, j : integer end; function b(i, j: integer) : a; begin b.i := i; b.j := j end;
I do not know if we want to implement that feature, but (assuming it is really accepted by BP) we should list it as known incompatiblity.