std.string.format results in run-time exception

[d coder]

Greetings

std.string.format throws runtime exception for BigInt and for BitArray
types even though writefln works fine. The run-time error says
std.format.FormatException at std/format.d(4744): Can't convert
std.bigint.BigInt to string: "string toString()" not defined

Here is a small test case.

Regards
- Puneet

void main()
{
  import std.stdio;
  import std.string;
  import std.bigint;
  import std.bitmanip;

  BigInt aa = 100;

  BitArray bb;
  bb.init([true, false]);

  writefln("%x", aa);
  writefln("%x", bb);
  writeln(format("%x", aa));
  writeln(format("%x", bb));
}
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