Absent the provided `opCast` overload for `BigInt`, the unittest I provided results in the following errors:
```
std/bigint.d(860): Error: template instance opCast!(const(BigInt)) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(860): Error: template instance opCast!(immutable(BigInt)) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(860): Error: template instance opCast!(BigInt) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(860): Error: template instance opCast!(immutable(BigInt)) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(860): Error: template instance opCast!(BigInt) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(860): Error: template instance opCast!(const(BigInt)) does not match template declaration opCast(T : bool)()
std/bigint.d(860): Error: function expected before (), not t1.(__error) of type _error_
std/bigint.d(880): Error: template instance opCast!(BigInt) does not match template declaration opCast(T : bool)()
std/bigint.d(880): Error: function expected before (), not n.(__error) of type _error_
std/bigint.d(881): Error: template instance opCast!(const(BigInt)) does not match template declaration opCast(T : bool)()
std/bigint.d(881): Error: function expected before (), not n.(__error) of type _error_
Failed: 'dmd' '-main' '-unittest' '-cov' '-v' '-o-' 'std/bigint.d' '-Istd'
```

... where 880 and 881 are the lines of the two casts.