Code shared from run.dlang.io. Run with '-unittest'

main.d

    struct IndexOfResult
    {
        ptrdiff_t value;
        alias value this;

        bool opCast(T : bool)() const @nogc pure @safe
        {
            return value >= 0;
        }
    }

void main()
{
  import std.string : indexOf; 
  import std.stdio : writeln, writefln;
  enum chr = 'a';
  enum arr = "dlang";
  if(auto res = IndexOfResult(arr.indexOf(chr)))
  {
    assert(arr[res] == chr);
    res.writefln!"[ --> %s ]";
  } 
  else 
    writeln("Not found!");
}