Question about std.conv.parse

[Jacky via Digitalmars-d-learn]

Hello everyone.I'm a newbie on the D language.When i use the 
library 'std.conv' ,i met some problem.
This is what I have:

    static import std.conv;
    string aaa = "123456789";
    uint idx = 5;
    string bbb = aaa[0 .. idx];
    uint work = std.conv.parse!(uint)(bbb); // this works

    uint didnotwork = std.conv.parse!(uint)(aaa[0 .. idx]);   
//but here's a error
    //template std.conv.parse cannot deduce function from 
argument types !(uint)(string)

So my questions are:
1) What is the difference between these two lines?
2) How to correct the second without assign a new variable?

Cheers,
Jacky