On Tue, 10 Dec 2019 15:54:52 -0500, Steven Schveighoffer wrote:

I have a feeling it's because your isim function is not declared
@safe. Because it's not a template, it doesn't get inferred safety. I
think this then cascades to some code that is being generated by the
diet parser (i.e. the __lambda1).

Try marking it safe and see what happens.

Marking this function as safe solved the problem. Thanks.

After necessary corrections, the source code of module hayvan.kedi is as follows. 

module hayvan.kedi;

string isim() @safe
{
    return "Minnoş";
}