Why does this work?

[h_zet via Digitalmars-d-learn]

import std.typecons;

auto foo2(R)(R foopara){
     return tuple(foopara, is(R==int));
}

void main(){
     auto tuple(a,b) = foo2(1);
}


I'm expecting some error such as can not act as left value but 
when I compiled this, no error occured. DMD version is DMD64 
v2.065.(ldc2 exited with error function declaration without 
return type)

Why does this work? Or it is a bug?