std/process.d: nothrow functions which throw (in struct ProcessPipes)

[Drone1h via Digitalmars-d-learn]

Hello All,

In phobos/std/process.d, in the ProcessPipes struct, we can see a 
few functions which are marked with "nothrow", but which (under 
some conditions) throw:

    @property File stdout() @safe nothrow
    {
        if ((_redirectFlags & Redirect.stdout) == 0)
            throw new Error("Child process' standard output 
stream hasn't "
                            ~"been redirected.");
        return _stdout;
    }

Would it be possible to explain this, please ? Or point me to 
some relevant documentation, please ?

Thank you.