__traits(compiles) + mixin

[cal]

I'm confused about this:

import std.conv;
void main()
{
    enum s = "`1`.to!int;";
    enum c = __traits(compiles, mixin("{auto a = new "~s~";}")); 
// line 1
    mixin("auto a = "~s~";");                                    
// line 2
}

This does not compile, giving errors about instantiating 
std.conv.to!(int).to!(string). If I switch the order of lines 1 
and 2 it compiles. Is this a bug or something I don't understand 
about __traits(compiles)?