BitArray's opCast does not work with const/immutable instances:

import std.bitmanip : BitArray;

void main() {
    BitArray m;
    const numbers = cast(size_t[]) m;
    const bytess = cast(void[]) m;
    
    // The following casts don't compile
    const BitArray c;
    const numbers = cast(const size_t[]) c;
    const bytess = cast(const void[]) c;
    
    immutable BitArray i;
    const numbers = cast(const size_t[]) i;
    const bytess = cast(const void[]) i;
}