>Nothrow and Pure are not part of the type signature of functions

Actually they are. Take a look at this example:

void main() {

    int delegate(int) @safe nothrow @nogc pure f;
    //This looks like something bad is happening, but the delegate literal 
    //actually has the proper attributes inferred so it Just Works
    f = delegate(int) { return 0; };

    int delegate(int) g;
    f = g; //Fails, as it should
}